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THE FLIGHT OF BIRDS 






































































































































PLATE 



Fro nils d ip nr 1 










THE 


FLIGHT OF BIRDS 

By F.' W.* HEADLEY, M.B.O.U. 

It 

Author of “ The Structure and Life of Birds ” 

“ Life and Evolution ” &c. 


WITH SIXTEEN PLATES 
AND MANY TEXT-FIGURES 


LONDON : 

WITHERBY & CO. 

NEW YORK J 

CHARLES SCRIBNER’S SONS 
1912 . 


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PREFACE. 


The flight of birds cannot but be of interest to 
men who sail the air on their biplanes or monoplanes, 
for the bird is, as yet at any rate, peerless among 
aviators, and in describing his methods I have kept 
in view the methods and the difficulties of those 
who are striving to rival him. It is possible, there¬ 
fore, that this little book may find readers among 
those who not only study flight but fly. But, since 
I have tried as far as possible to avoid technical 
terms and make it easily intelligible, I hope it may 
appeal to the larger class who, whether scientific 
ornithologists or not, take a delight in birds and 
their doings. 

I have to thank many friends for their help ; Mr. 
J. A. Tregelles for three drawings which can speak 
for themselves (the frontispiece and figures 13 and 14) 
and also for reading a typewritten copy of the 
book : Mr. C. W. Adams for reading the first five 
chapters : four Haileyburians past or present for 
drawings and a photograph ; W. T. Hichens for 
figures 18-23 ; G. G. Nanson for the diagrams ; 

V 



G. H. G. Perry for figure 27 ; A. P. Whitehead for 
Plate xiv, D. Mr. P. Disney has been kind enough 
to read the proofs. In addition I have to thank 
Mr. E. 0. Gilson for some valuable criticisms, Mr. 
F. H. Jeffery for help in some small experiments: 
Messrs. Duckworth have very kindly allowed me 
to reproduce several of the illustrations of my Life 
and Evolution. The photographs (with the one 
exception I have mentioned) are my own. Most of 
them are now published for the first time, but 
several have appeared in British Birds , and three 
in the Journal of the Aeronautical Society. The 
Editors of British Birds and the Secretary of the 
Aeronautical Society kindly permit me to use them. 

F. W. HEADLEY. 


Haileybury, 

March, 1912. 


VI 


CONTENTS. 


CHAPTER I. 


Gliding. 


PAGE 


Resistance of Air—Lift and Drift—Curve of Wings—Area 

of Supporting Surface .. .. .. .. .. 1 


CHAPTER II. 

Stability. 

Centre of Gravity—Points that make for Automatic Stability 

—Voluntary Adjustments .. .. .. .. 23 


CHAPTER III. 

Motive Power. 

Leverage—Propulsion—Phases of the Wing-stroke .. 38 

CHAPTER IV. 

Starting. 

The Preliminary Jump—Loss of Altitude between Wing- 
strokes—The Wing’s Freedom to Rotate—Aeroplanes 
—Muscles—Big Birds and Small .. .. 48 


CHAPTER V. 

Steering. 

A Variety of Methods—Good Steerers and Bad .. .. 59 


CHAPTER VI. 
Stopping and Alighting 


.. 64 



CHAPTER VII. 

The Machinery of Flight. 

The Breastbone and the Connected Bones — Muscles and 
Quality of Muscle—The Scaffolding of the Wing—Pneu¬ 
matic Bones—Stiffness of Wing—Expanse of Bone— 

The Spreading of the Wing—Structure of a Flight- 
Feather—Moulting—Legs .. .. .. 67 


CHAPTER VIII. 

Varieties of Wing and of Flight. 

Curve—Narrow and Broad Wings—Styles of Flight—Flight 

in Flocks—The Whir of Wings . . .. .. .. 91 

CHAPTER IX. 

Pace and Last. 

Experiments and Observations—Wind—Velocity of Migra¬ 
tory Flights—Endurance .. .. .. .. 103 

CHAPTER X. 

Wind and Flight, 

Rising—Flight with the Wind—Undulating Flight 
without Movement of the Wings—Advance in a Direct 
Line without Movement of Wing—Advance Sideways 
in a Direct Line—Soaring—Soaring in a Horizontal 
Wind Impossible.. .. .. .. .. .. 120 


CHAPTER XI. 

Some Accessories. 

Digestion—Circulation, Breathing, Temperature—Repair 

of the Machine—Call-notes and Song .. .. .. 154 


viii 


LIST OF PLATES. 


I.—Eagles soaring .. .. .. .. Frontispiece 

Facing 

page 

II.—Photographs of Pigeons and Gull, showing right 

and left wings giving different strokes .. .. 30 

III.—Photographs of—A: Pigeon using tail for mainten¬ 
ance of equilibrium. B : Pigeon gliding. C : 

Gulls gliding .. .. .. .. .. 36 

IV., V., and VI.—Photographs of Pigeons showing the 

phases of the wing-stroke in series .. 40, 42, 45 

VII.—Photographs of Gulls flying .. .. .. 46 

VIII.—A and B : Photographs of Pigeons starting to fly. 

C : of Pigeon rising with body steeply inclined. 

D : of Herring-Gulls starting . . .. .. 49 

IX.—Photographs showing different methods of steer¬ 
ing—A and B : of Gulls. C and D : of Pigeons 60 

X.—A and B : Photographs of Pigeons alighting. C : 

of Pigeon checking speed .. .. .. 64 

XI.—A : Photograph of wing, showing elastic liga¬ 
ment. B : of flight-feathers and tail-feather .. 85 

XII.—Drawing by I. C. Maclean (from Life and Evolu¬ 
tion), showing minute structure of flight-feather 86 

XIII., XIV., XV.—Photographs of different types of 

wing .. .. .. .. .. 91, 92, 94 

XVI.—Photographs of Gulls with motionless wings fol¬ 
lowing steamer .. .. .. .. .. 136 


IX 



LIST OF FIGURES IN TEXT. 


Page 

1. —Glass balls, an illustration of Sir I. Newton’s experi¬ 

ment .. .. .. .. .. .. .. 3 

2. —Diagram illustrating gliding .. .. .. .. 6 

3. —Parallelogram of forces .. .. .. .. .. 7 

4. —Diagram and drawing of boat tacking .. .. 8 

5. —Illustrating kite-flying .. .. .. .. .. 9 

6. —Showing the proportion of resistance to support .. 10 

7. —Further illustrating the subject of resistance and 

support .. .. .. .. .. .. .. 11 

8. —Planes set at different angles to the horizon .. .. 12 

9. —Showing the advantage of a curved surface .. 16 

10. —Two cubes .. .. .. .. .. .. 19 

11. —Showing the disadvantage of an excessive curve .. 25 

12. —Showing the shifting of the centre of pressure .. 26 

13. —Swift gliding with wings fully extended .. .. 35 

14. —House-Martin gliding with wings partly flexed .. 35 

15. —Showing the velocity with which the extremity of the 

wing moves .. .. .. .. .. .. 39 

16. —Showing how the downstroke propels as well as lifts 42 

17. —(After Marey) Gulls flying .. .. .. .. 43 

18. —Breastbones of Guillemot and Falcon .. .. .. 68 

19. —Breastbone and connected bones of Adjutant .. 69 

20. —Clavicles of Tern and Eagle .... .. .. 70 

21. —Humerus of Eagle .71 

22. —Humerus of (a) Skua ; (6) Hombill; (c) Sea-Eagle. 

Drawn to scale.. .. .. .. .. .. 78 

23. —Skeleton of wing of Adjutant .. .. .. .. 80 

24. —(After Alix) showing elastic ligament, anterior wing- 

membrane, etc .. .. .. .. .. .. 82 

25. —Illustrating advance with motionless wings at right 

angles to the wind .. .. .. .. .. 131 

26. —Illustrating advance in the teeth of the wind with 

wings held rigid .. .. .. .. .. 136 

27. —Gull gliding sideways with wings held rigid .. 140 


x 












THE FLIGHT OF BIRDS. 


CHAPTER I. 

GLIDING. 

RESISTANCE OF AIR—LIFT AND DRIFT—CURVE OF WINGS 

AREA OF SUPPORTING SURFACE. 

Not long ago the sight of a sparrow on the wing, 
or even the sight of a lark rising in air and singing 
as he rose, excited but little interest or wonder in 
the mind of the average man. It came natural 
to birds to fly, and to fly so perfectly that they 
concealed their art. If they had had difficulty in 
flying, there would have been matter for astonish¬ 
ment. But why waste any wonder on the easy 
flight of a lark or a sparrow ? Such familiar things 
were taken for granted and seemed to call for no 
explanation. As soon, however, as men began to 
emulate birds—gliding downwards first from some 
elevated position and, later, by means of powerful 
engines, rising from the ground—then there was 
less disposition to take the bird’s flight as a matter 
of course. The bird too, it was felt, had once 
been a beginner. He too had had difficult problems 
to solve and had solved them long ago, the very 
perfection of his methods making it difficult to 


2 


THE FLIGHT OF BIRDS 


understand fully what his methods are. But what¬ 
ever the difficulty of learning from him, the bird had 
certainly much to teach. Such a past master in 
the art of flying must needs be able to give some 
hints to the man whose ambition it was to discover 
the ideal design for an aeroplane. 

And so the question “ How does a bird fly ? ” 
became one that had an interest not only for 
ornithologists. Those who gave it any thought 
soon found that it involved a number of problems. 
First comes the question how the yielding air can 
support a heavy body when gravity is tugging it 
downwards. Then, of no less practical interest 
is the question, how the bird maintains his equi¬ 
librium, or recovers it if for the moment he happens 
to lose it. How does he move his wings so that 
they may at once propel and support him ? How 
does he steer ? How is it that the small bird is 
able to start so easily from the level ground ? 
What of the easier but important problem of alighting 
without jar ? How is it that the bird, big or small, 
is able to treat with contempt the sudden gusts 
and eddies that the boldest aviator fears ? Does 
the bird ever gain advantage from the unequal 
velocity of the wind ? Does he search for up-currents 
and get them to lift him ? 

Resistance of Air. 

The first of these problems was solved by Sir 
Isaac Newton. By means of an experiment, that 
may well astonish us if we bear in mind how 
imperfect were the appliances that he had at his 
disposal, he demonstrated the peculiar property of 


GLIDING 


3 


air that makes flight a possibility. The resistance 
of air to a moving body may be little, may be 
great. That it may sometimes be considerable 
many a bicyclist has found out to his cost when 
he has tried to double and redouble his speed. 
There comes' at last a time when the “ yielding 
air ” almost refuses to yield. In fact the resistance 
it offers to a body moving through it increases as 
the square of the velocity. Of the ingenious experi¬ 
ment by which Newton proved this I must give 
a brief account. He took glass globes of equal 
size but unequal weights, corresponding to the 
figures 1, 4, 9, 16. These he let fall from the dome 
of St. Paul’s and measured the velocities when 
they had settled down to a uniform pace. Since 
there was no gain or loss of velocity, the resistance 
of the air must have been in each case equal 
to the weight of the falling globe. But it 



12 3 4 


Fig. 1. 

turned out that the relative velocities corresponded 
to the figures 1, 2, 3, 4, whereas the weights of the 
globes are represented by the squares of these 
numbers, viz. 1, 4, 9, 16. From this he concluded 
that the resistance of the air increases as the square 
of the velocity. Recent experiments have shown 
that Newton’s law is not absolutely accurate. Up 






4 


THE FLIGHT OF BIRDS 


to 10 metres per second (about 22 miles per hour) 
the increase in resistance is rather less than the 
square of the velocity, whereas for velocities greater 
than 10 metres per second the resistance increases 
at a still more rapid rate.* 

But to resist, to give effective support, the air 
must be in proper condition. If it has just been 
pounded and battered, it is useless to trust to it. 
And so perpetual forward movement to ever fresh 
fields is necessary. A bird cannot mark time in 
the air. It is true that one often sees a Kestrel 
Hawk hovering, apparently without any forward 
motion, his wings, one might imagine, pounding 
the same air over and over again. But there is 
good reason to believe that the Kestrel never hovers 
except when there is a fair breeze to bring fresh 
unbattered air to his wings. If a long string be 
tied to a Pigeon’s leg, he will fly perfectly well till 
he reaches the end of his tether. But as soon as he 
feels the pull of the string he will drop to the 
ground. When a flock of birds are travelling across 
the sky, it is easy to see that not one of them puts 
himself immediately behind any other. Were any 
individual to do so, he would not have at his disposal 
the fresh undilapidated columns of air that are 
essential, and moreover he would feel all the back¬ 
wash from the bird in front. At least one aviator 
has lost his life through getting into the wash of 
an aeroplane that was a little ahead of him. In 
building a biplane the question of finding air that 
has not been tumbled is one that cannot be neglected. 
A writer who appears to speak with authority says 
* Marey, Vol des Oiseaux , p; 218. 


GLIDING 


5 


that its two surfaces must have a space of four 
feet between them, or else they will interfere with 
one another ; one will tumble the other’s air. It 
is true that I have seen a triplane with its “ planes ” 
at a distance from one another of three feet only. 
But whatever the minimum interspace, it must be 
considerable. And yet when birds are flying low 
over water they do not ruffle its surface. Pelicans 
flying over the Nile, not more than a foot from the 
water, so a good observer says, left the surface 
undisturbed.* Still, I cannot help thinking that 
the space between the birds and the water was 
under-estimated. At any rate a pigeon’s first 
strong wing-strokes when he rises scatter straws, 
dust, and feathers from the ground where he takes off. 

Enough has now been said to show how effective 
is the support that the air can give to a rapidly- 
moving body, and how essential it is that the bird 
or the aeroplane should be unceasingly moving 
onward to columns of air that are fresh and 
undisturbed. 

Lift and Drift. 

It is probable that the power of flight was first 
attained by terrestrial birds which jumped from 
tree to tree, their wings aiding them to float through 
the air. If this floating through the air had been 
their crowning achievement, they would have done 
nothing remarkable, for mere gliding is not flight. 
Since, however, gliding seems to have been their 
first attainment, just as it has been with human 
flyers, I shall begin by investigating this compara¬ 
tively simple performance. At the outset there 
* Quoted by Marey, Vol des Oiseaux, p. 30. 


6 


THE FLIGHT OF BIRDS 


is a very elementary matter to be made clear. Let 
us picture the bird to ourselves as gliding in an 
exactly horizontal direction. The resistance of the 
air to his forward progress is equivalent to a 
horizontal wind blowing against him, and what 
is wanted is support, to counteract the downward 
pull of gravity. From a horizontal wind he must 
somehow get this support. The question presents 
no difficulty to anyone who has studied elementary 
mechanics, and in the application of mathematical 
principles the bird is a wonderful proficient. He 
inclines his aeroplane (his expanded wings and his 


W 

body) slightly upward, and the result is that the 
air supports him more than it resists him. It acts 
at right angles to the plane that he opposes to it. 

In fig. 2, b represents the gliding bird, w the 
rush of air against his expanded surface. If the air 
is still, there will, nevertheless, be the wind due to 
the bird’s own velocity. W', at right angles to the 
bird, shows the direction in which the wind acts ; 
its lifting power, when the bird inclines himself 
thus, being greater than its resistance. The action 
of the air at right angles to a plane moving through 
it, is illustrated by an experiment sometimes made 
by very unscientific persons. A stone is thrown 



Fig. 2. 



GLIDING 


slanting-wise at a window—thrown by a dexterous, 
mischievous urchin standing far off beneath the 
wall of the house—and the fragments of glass as 
they go flying into the room make a right angle 
with the plane of the window. 

The air, then, striking against the plane presented 
by the bird acts at right angles to it, and there 
comes into play a resolution of one force into two. 
This introduces what is known as the parallelogram 
of forces. 

There is a force acting along a b (fig. 3), and if 
resistance is in this direction it may resolve itself 



Fig. 3. 


into two forces represented in magnitude and direc¬ 
tion by the lines c b, d b. Take the case of a boat 
tacking. The wind acts at right angles to the sail, 
but the boat refuses to move much in that direction 
(i.e. broadside on) ; she makes only a little leeway. 
The force of the wind, therefore, acting towards x 
(fig. 4), is broken up into two forces acting towards 
d and l, and that towards l, as I have said, does not 
count for much, since the boat will not readily move 
broadside on. So there is much headway and a 
little leeway. The principle at work is the same 
when a bird is gliding horizontally. His body is 
inclined slightly upward. The force of the air acts 




8 


THE FLIGHT OF BIRDS 


at right angles to the surface he presents to it. 
And this force is broken up into two, the one 
supporting him, the other tending to drive him 
backward, or, as it is now briefly and clearly put, 
the force is resolved into lift and drift. 



Fig. 4. 

Boat tacking.—w, the wind which acts at right angles to sl 
(the sail), towards x. The force is broken up into two forces, 
acting towards d and l. 

A paper kite supplies us with another and perhaps 
still more apt illustration. If the air is still, the 
kite-flyer may supply a force by running with the 
string ; if there is a wind, he has merely to hold 
the string. Let us imagine that the wind is blowing 
horizontally. It will, nevertheless, lift the kite, as 
long as the string is held firmly. It strikes against 
the oblique surface which the kite presents, and 








GLIDING 


9 


acts at right angles to it. Drift is prevented by the 
firmly-held string, and lift is the sole result (fig. 5). 

It is only when there is resistance that there is 
any force to be thus resolved into two. If the 
string breaks, gravity at once begins to pull the 
kite to the ground. 

We have now a further question to investigate 
and, to simplify it, we must consider not the gliding 
bird, but a flat plane set at a slight upward incline 
and driven horizontally through the air. A flat 
plane, having none of the curves and concavities 
of the bird’s wing, is far inferior for purposes of 
flight, but its simplicity recommends it when the 



Fig. 5. 

To Illustrate Kite-Flying. 

w, horizontal wind blowing against kite, k t. w' (at right angles 
to k t), line along which the force of the wind acts. 

object is to explain elementary principles. Set at 
an incline and moving horizontally it will tend to 
rise. The resistance of the air is equivalent to a 
wind blowing against it. The wind would act in 



10 


THE FLIGHT OF BIRDS 


a direction at right angles to the plane, and the 
force so acting is, of course, resolved into two, one 
tending to raise the plane, the other resisting its 
horizontal progress. The question, then, which we 
wish to decide is : In what proportion is the force 
of the wind divided between the two components, 
between lift and drift ? Now, supposing that b d 
(fig. 6) represents the plane set with an upward incline 
and driven horizontally through the air, it can be 
shown that line d c represents the resistance of the 



Fig. 6. 


air to its onward progress and b c, a much longer 
line, the support given by the air. In fact, when 
the plane is inclined but slightly upward the support 
it gets from the air is far greater than the resistance, 
a fact that can be proved by experiment. The 
mathematical proof that n c represents the drift 
(or resistance), and b c the lift, I give in fig. 7. 

It is now apparent that, as the angle of inclination 
to the horizon is more and more reduced, the pro¬ 
portion of lift to drift becomes greater and greater. 
Why not, then, reduce the angle till the resistance 
of the air to horizontal progress becomes a negli¬ 
gible quantity ? But obviously there is a limit 
to the process. If the plane has so slight an incline 
that it is almost horizontal, the air will offer but 
little resistance, and however big a proportion of 
this we may allot to lift and however small a one 




GLIDING 


11 


to drift, yet the actual amount of lift will be but 
small. If the strongest man among the survivors 



b d is the plane, w the wind blowing horizontally against it. 
b c is the cosine of the angle at b, d c the sine of the angle. Pro¬ 
duce c d to e, making d e — b c. From d draw d f at right 
angles to b d. From e draw e f at right angles to d e. (This 
decides the length of the line d f). Draw f g parallel to ed 
and d g parallel to f e. Let d f represent the force of the wind 
acting at right angles to b d. It can be resolved into two forces, 
f e and f g ( =e d). e d we know= b c, and it can be shown 
that f e = d c. 

In the triangles d e f, b c d, b c = d e, and the angles at e 
and c are each right angles. If we could prove a second angle 
= a second angle, the triangles would be equal in every respect. 
Now the angles at d together —two right angles, and the angle 
b d f is a right angle. Therefore b d o with f d e makes one 
right angle. But d f e with f d e makes one right angle. There¬ 
fore angle d f e = angle b d c. The triangles then are equal, 
and the two sides f e and d e = respectively d o and b c. But 
f e represents drift and d e lift. Therefore d c, the sine of the 
angle at b, represents the drift, while b c, the cosine, represents 
the lift. 


of a starving ship’s crew is able to take for himself 
nineteen-twentieths of the last biscuit, nevertheless 
he gets but a poor meal; and it is obvious that if 








12 THE FLIGHT OF BIRDS 

the plane were not inclined at all, but presented 
its edge to the air, there would be practically no 
support. There is, therefore, beyond all dispute 
a limit somewhere to the possible reduction in the 
incline of the aeroplane. The question is where, 
for practical purposes, that limit comes in. Newton 
formulated a law with regard to this, a law which 
is now quoted only to be condemned, and some¬ 
times quoted with expressions of contempt for 
him and mathematicians in general.* Newton 
held that the resistance of the air increases as the 
square of the sine of the angle of inclination. 



Fig. 8. 


Thus, if we take angles of 5°, 10°, 20°, the resistance 
would increase, from 25 to 100, to 400. Instead of 
being grateful to Newton for his great contribution 
to our understanding of flight, his discovery that 
the resistance of the air increases as the square 
of the velocity of bodies moving through it, some 
writers have depreciated him and his work because 
he has come to a wrong conclusion on this further 
question. As a matter of fact the resistance of 
the air varies as the angle, i.e. as its sine, not as 
the square of its sine. Therefore, as you diminish 
the angle, you still have a considerable amount of 
resistance, and it is divided up largely in favour 

* See Sir H. Maxim’s Artificial and Natural Flight, pp. 2-6. 
The question is well dealt with in Prof. Langley’s Experiments 
in Aero-dynamics ; see especially pp. 24 and 25. 






GLIDING 


13 


of support, of lift as opposed to drift. If Newton’s 
law held good, if the angle of inclination were more 
and more reduced till it amounted only to, say, 5°, 
then the resistance offered by the air would be too 
small to be worth dividing up between lift and drift. 
Even the lion’s share would be worth next to 
nothing. But experiment shows that an aeroplane 
set at an angle of 5° can, if it travels fast, find 
support in the air. 

But, although as we reduce the angle of inclina¬ 
tion the resistance of the air does not diminish at 
the rapid rate that Newton imagined, nevertheless 
there must obviously be a point beyond which the 
fining down of the angle cannot go, since the air 
will at last cease to give the required support. 
But before we reach the lowest possible limit another 
factor comes in which checks us as we are making 
successive reductions. As we continue to cut 
down the angle there comes at last a point at which 
the question of friction obtrudes itself in very 
unpleasant fashion. Imagine the aeroplane driven 
through the air at a very minute angle. If it is 
to find support, it must travel at a very great pace, 
else the resistance of the air will be too small. 
With every diminution of the angle there must 
be an increase of pace, and it might be thought 
that, if only the pace were increased sufficiently 
to make up for the diminution of the angle, all 
would go well. Professor Langley made some 
most valuable experiments which showed the great 
advantage of a small angle of inclination, and, 
emboldened by this great and important discovery, 
he proceeded to frame a formula and to speak 


14 


THE FLIGHT OF BIRDS 


of a law. By experiment he had discovered an 
indubitable principle, but subsequent experiments 
have discovered an equally indubitable fact, which 
at a certain point interferes with its operation. 
The angle cannot be reduced below, approximately, 
5° without bad results. At a less angle, with the 
necessary increase of pace, the friction of the plane 
against the air increases so rapidly that, so far from 
there being any gain from the further reduction 
of the inclination, there is an actual loss.* 

On this subject a writer in Flight has some very 
interesting calculations, founded partly, it is true, 
on theory, and requiring further verification by 
experiment, but probably representing the facts 
without any considerable deviation.! Indeed experi¬ 
ment has already proved his main thesis, so that 
it is only detail that requires further testing. He 
imagines aeroplanes having the ideal camber or 
curve for their planes, a large curve if they are to 
travel slowly at a great angle of inclination, a slight 
curve if they are to travel fast. They are to carry 
a weight of 100 lb. Let us imagine them driven by 
3 horse-power at the angles 30°, 25°, 20°, 15°, 10°, 5°. 
The velocity will increase rapidly as the angle is 
reduced. At 30° it will be 38 miles per hour. 
With the successive reductions of the angle it will 
increase to 47, 60, 78, 106, 134. The pace at 5° 
and even at 10° is greater than most people would 
wish to travel at, so the power applied might with 
advantage be reduced. Two horse-power at 10° 

♦See The Aero Manual , p. 21; Flight , July 9, 1910; and 
Langley’s Aero-dynamics , p. 37. 

f Log. cit. 


GLIDING 


15 

and 5° respectively would mean 70 and 90 miles 
per hour. Even 1 horse-power, with an angle of 
5° gives a velocity of 43; in other words, 1 horse¬ 
power, if the plane is set at an angle of 5°, is more 
effective than 3 horse-power when the angle is 30°. 
In fact, if you have a good aeroplane and the skill 
to use it well, you require less power to fly fast than 
to fly slow. But if you reduce the angle of inclina¬ 
tion below 5° you find that the tables are turned 
upon you. So far from economising power, you 
would have to use it in lavish style, to overcome 
friction. 


Curve of Wings. 

We must now turn to the question of curves. 
The curved surface is undoubtedly superior to the 
flat. A toy paper glider, being a mere feather¬ 
weight, can dispense with curves, but without 
curved surfaces—cleverly designed ones too— 
aviation would be out of the question. An aeroplane 
presents concave surfaces to the air ; its “ planes ” 
curve from front to back, and a bird’s wings have 
concavities that are probably better adapted for 
flight than anything that human ingenuity has 
designed. On this subject even an umbrella can 
tell us something. When its ample concavity is 
turned towards the wind its efficiency in catching 
and holding the air is so great that it speedily 
becomes a wreck of wires. Lilienthal, a distin¬ 
guished flight pioneer, whose experiments in gliding 
did much to make aviation practicable, fully ap¬ 
preciated the value of the curved surface. Some¬ 
times when he was carrying his glider to a little 


16 THE FLIGHT OF BIRDS 

hillock, his jumping-off place, outside Berlin, he 
was cheered and emboldened by the way the wind 
would catch his glider’s well-designed concavities 
and nearly lift him from the ground. When he 
reached his little hillock he would jump from it 
and make glides of 150 yards and more. 

Lilienthal found that the air that met his glider’s 
concave surface did not act at right angles to the 
chord of the arc (fig. 9), along f, but along a 
line occupying somewhat the position of f'. 



The arrow shows the direction in which the glider is travelling. 


Experiments made by Mr. Wilbur Wright and 
his brother have shown that Lilienthal had called 
attention to a principle on which the aviator can 
base his calculations. In short, with a properly 
rounded surface the lift is greater and the drift is 
less than with a flat one. In order to obtain the 
maximum gain, the best possible curve must be 
discovered, and this can only be done by experiment. 
The writer in Flight already quoted is of opinion 
that each velocity has its appropriate curve. An 
aeroplane that is to be a racer should, he maintains, 
have its surfaces much less curved than a slowly- 
flying one, and undoubtedly the wings of birds 
suggest that he has enunciated a true principle. 




GLIDING 


17 


Very fast flyers, such as the Swift, have very little 
concavity. There is another fact, too, that points 
to the same conclusion : in a bird’s wing the part 
that is near the body is the most concave, while 
towards the extremity it becomes nearly flat, in 
some cases quite flat. Now the extremity of the 
wing, when a stroke is given, moves with great 
velocity forward as well as downward, the near 
part comparatively slowly. Thus birds seem to be 
exponents of the principle that the curve should 
vary inversely as the speed. And here I must 
point out a great advantage the bird has over his 
upstart rival, the aeroplane. The aeroplane is 
rigid, with at most the rear edge only of its plane 
flexible. If the principle to which I have called 
attention is sound, it must be built either with a 
curve appropriate to great speed or with one which 
suits comparatively slow travelling. It cannot 
be varied during flight according as the pace is 
varied. But when the bird takes a very hard 
stroke, the front-to-back curve of his wings, which 
is mainly the curve of the feathers, is much reduced, 
so that by plying his wings with great vigour he to 
some extent modifies his configuration and adapts 
it to his high velocity. (For birds in rapid flight, 
see Pis. iv-vii.) 

It is always well to ask “ Why is it so ? ” when¬ 
ever experiment discovers a fact. Why, then, 
with a curved surface is the lift more and the drift 
less ? It is very difficult to see how the air does 
its work, and theory on such a subject may prove 
to be only random guesswork. But the lift would 
seem to be greater from the simple fact that the 


18 


THE FLIGHT OF BIRDS 


air cannot so easily escape. When the moving 
surface is a plane, the air is unconfined and the 
force is dissipated. The reduction of the drift 
(i.e. of the resistance to the forward movement of 
the surface) may be illustrated by the passage of 
a boat, built on good lines, through the water. The 
resistance of the water to the bow is balanced, or 
nearly so, by the shove given to the stern by the 
water closing behind it, so that there is little, except 
friction, to retard the boat. In the same way the air 
closes upon the hinder part of the curved surfaces 
of an aeroplane ; there is no region of “ dead air ” 
behind it. The matter is well explained in a little 
book on Model Flying Machines (pp. 19 and 20) 
by Mr. W. G. Aston. There is a region of “ dead 
air ” behind a flat surface when it is given an upward 
incline and is driven forward horizontally. The 
amount of “ dead air ” is the measure of the amount 
of resistance. 

Area of Supporting Surface. 

We can hardly leave the subject of gliding without 
touching on this important question : What area 
of supporting surface is required for, say, one pound 
weight ? Obviously no hard and fast rule can be 
laid down. Since it is the front part of the plane 
on which the wind mainly impinges, the back part 
is of less importance ; indeed it may be quite 
superfluous, a useless encumbrance, if the breadth 
from front to back is excessive. Besides this, as 
I shall show, we cannot frame a formula that will 
apply equally to big aeroplanes and small, to the 
big bird and the small bird. The Aero Manual 


GLIDING 


19 


says, “ Nearly one square foot for each pound 
weight.” But it has in view big machines only. 
When we go to the various creatures that fly, weigh 
them and measure their surfaces, we get most 
diverse results. The big flyers, we find, have small 
wings, the small flyers big ones, if difference in weight 
is allowed for. Compared with a gnat or a butterfly 
a Stork has a very small supporting surface, a small 
one even when he is compared with a Swallow. 
When M. de Lucy discovered these facts and pub¬ 
lished them the astonishment was great. Here 
are some of his figures :— 


Gnat 


Surface per lib. 
sq. yds. ft. 

. . 4 6 

weight. 

ins. 

105 

Butterfly.. 

.. 

.. 3 

8 

87 

Swallow .. 

.. 

.. 0 

4 

18 

Pigeon 


. . 0 

1 

14 

Stork 


.. 0 

0 

122* 



But before long mathematicians hit upon a plan 
by which they were able (or thought they were able) 
to rob the figures of their startling character, and 

* See Marey, Animal Mechanism , p. 222, and Pettigrew, Animal 
Locomotion , p. 133. 










20 


THE FLIGHT OF BIRDS 


put big and small birds in this respect more or 
less on a level. They appealed to the elementary 
principles of geometry. If you take two cubes, 
a side of one of which is twice the length of the 
other, the larger one is in bulk eight times as great 
as the smaller one, but its surface area is only four 
times as great. This holds true of other figures 
of three dimensions that are not cubes. Magnify 
a bird till it is eight times its former bulk and you 
will only have multiplied its surface area by four. 
In order, then, to compare bird with bird correctly, 
you should take (so say these theorists), the cube 
root of its weight (for the weight is practically the 
bulk, i.e. three dimensions multiplied together) 
and the square root of its surface area (since that 
is two dimensions multiplied together). When we 
adopt this method we find the preponderance of 
the small bird in point of wing-area per pound 
weight not so very great. And we may, if we are so 
constituted, derive a certain comfort from feeling 
that we are following out geometrical principles. 
But these principles have, it must be owned, in the 
case of some species been widely departed from. 
What are we to make of the legs and neck of the 
Flamingo ? If any small bird of ordinary build 
were symmetrically enlarged, should we ever arrive 
at elongations so enormous ? It is, of course, true 
that if the bulk of a small bird were multiplied 
many times, and the area (not the bulk) of his wings 
increased in proportion, he would have a greater 
expanse of wing than his muscles could possibly 
work. If the Stork had a wing-area as great in 
proportion to his weight as the Swallow, then 


GLIDING 


21 


(taking his weight as four pounds and four-fifths) 
his wings would together measure twenty square 
feet! This would be a monstrous acreage of wing 
to raise and lower. But when we have pronounced 
it monstrous, we have still to answer the question 
why it is that the big flyer requires, in proportion 
to his weight, a comparatively very small support¬ 
ing surface ? 

Let us imagine the Swallow supplied with wing- 
area at no more liberal rate per pound weight than 
that at which the Stork is supplied. Then, taking 
the Swallow’s weight to be about five-sevenths of 
an ounce, he would have five square inches of 
wing-surface—two and a half on either side—a 
miserably poor allowance. A wing so small would 
be largely made up of margin, and the air would 
escape at the edges. The gnat has over four and 
a half square yards of wing for one pound weight. 
His actual allowance for his almost imponderable 
insignificance is considerable, but if we make pro¬ 
vision for him at the rate at which the Stork is 
supplied, his wing-surface becomes a mere point, 
“ without parts and without magnitude,” to quote 
Euclid’s familiar definition. The air would offer 
no resistance to so near an approximation to the 
theoretical point. Here, no doubt, we are getting 
at the main fact that explains the comparatively 
small size (when weight is allowed for) of the wings 
of the great flyers. Since the wing-surface is much 
larger absolutely the air does not escape so easily 
at the margins ; each square inch is more effective, 
since there is less waste. Later on I shall make 
a further comparison of big birds and small, but 


22 


THE FLIGHT OF BIRDS 


one important difference and its explanation have, 
I hope, been made clear (see Chap. iv). 

It will be more convenient to consider the question 
of upward and downward gliding in the next 
chapter. 


CHAPTER II. 


STABILITY. 

CENTRE OF GRAVITY—POINTS THAT MAKE FOR AUTOMATIC 
STABILITY—VOLUNTARY ADJUSTMENTS. 

There are two things that we must carefully 
distinguish—stability and the maintenance of equi¬ 
librium. An aeroplane, when it has once completely 
lost its balance, cannot recover it, though some 
swaying or pitching can, no doubt, be corrected. The 
stability of a bird is a very different thing from 
the aviator’s careful maintenance of equilibrium. A 
very strong and sudden gust may throw the bird on 
his side, or even on his back, and yet he will very 
quickly right himself. 

Centre of Gravity. 

But we want to know whether a bird in ordinary 
flight, when there are no very sudden gusts, is auto¬ 
matically stable, or whether he has to be perpetually . 
making small adjustments. It is sometimes main¬ 
tained that a bird need take no trouble about the 
question of balance, since his centre of gravity is 
low down. The great flight muscles, which are 
massed upon the breast, form a great part of his 
weight. I have weighed the three pairs of breast 
muscles of two Wood-Pigeons and those of two 


24 


THE FLIGHT OF BIRDS 


domestic Pigeons, and found that they account for 
about one-fifth, or even more than one-fifth, of the 
weight of the whole bird. Indeed, in one Wood- 
Pigeon they were equal to three thirteenths of the 
total weight—a little less than a quarter.* Hence 
the centre of gravity lies considerably below, though 
not so very far below, the shoulder joints. But the 
idea that it is good to have the centre of gravity of 
a flying machine low down is altogether a miscon¬ 
ception, a fact that can easily be put beyond all 
doubt by experiments with small gliders. Weight, 
placed low, tends to make the machine oscillate 
and even swing right round. It is best that it 
should be on a level with the supporting surface. 
In the case of the bird, the chief supporting surfaces, 
the wings, are always changing their relative position 
as they are raised or lowered, and, of course, as they 
move they to some extent raise or lower the centre 
of gravity. This must make voluntary adjustments 
for the purpose of recovering balance still more 
necessary. But Professor Marey has pointed out 
a curious fact which may make these oscillations 
less difficult for the bird to deal with. When the 
down-stroke takes place and the wings are lowered, 
the centre of gravity occupies a lower position in 
the bird, but the bird as a whole, except during 
very rapid flight, rises. With the up-stroke, on the 
other hand, there is a raising of the centre of gravity, 
but a lowering of the bird ; hence, though the bird’s 

* Legal and Reichel, in the Jahresberichte der Schlesischen 
Gesellschaft (1879), give 3.^3 ( = = more nearly -| than £) as 

the proportion of the total weight of the pigeon accounted for by 
the three pairs of breast muscles; But this would seem to be a 
mistake: Charadrius, according to them, comes next with f. 


STABILITY, 


25 


body rises and falls, its centre of gravity travels 
onward almost in a straight line.* 

Points that make for Automatic Stability. 

Though the position of the centre of gravity is 
of no avail, the general build of the bird and the 
elasticity of the feathers make for automatic stabil¬ 
ity. To begin with, there is the question of curves. I 
have already pointed out that a curved surface gives 
more lift and less drift than a flat one. But anyone 
who experiments with gliders soon finds out that 
an excessive curve is fatal to stability. In the 
preface to the Aero Manual (1910) it is stated that 
the curve (the depth of the concavity) should not 
be more than one-twelfth of the breadth. Imagine 
a surface with a more considerable curve. The wind 



W 


Fig. 11 . 


Diagram to show the disadvantage of an excessive curve. 

will impinge upon its upper side and the glider will 
duck and descend rapidly to earth. But if the curve 
is only slight, though the tendency to duck and dive 
may, no doubt, arise, yet it tends to correct itself. 
Imagine the glider launched on its way and develop¬ 
ing a tendency to lower its head and raise its tail 
unduly. In proportion as the tendency develops, in 

* There is no need to discuss the question of pendulum stability. 
It is possible to suspend a heavy weight from an aeroplane, and 
there are theorists who hold that automatic stability may be thus 
attained. It is obvious that in the case of a bird nothing of the 
nature of a pendulum is possible. See Flight, Feb. 25, March 18, 
and March 25, 1911. 



26 


THE FLIGHT OF BIRDS 


proportion as the glider makes a smaller and smaller 
angle with the horizon, the point at which the air 
acts will be progressively nearer to the front margin, 
and this will obviously tend to prevent a dive 
downward. 



Fig. 12 

Diagrams showing the shifting of the centre of pressure. 


n 


This point requires some explanation. Take as 
an illustration a sailing boat when it is tacking. 
When it sails close to the wind, the air which strikes 
against the forepart of the sail is deflected from its 
course, and rushing towards the stern of the boat 
forms a buffer which shields the rest of the sail from 
the air, which would otherwise have impinged upon it. 
The drawing of a boat tacking, on page 8, and 
the accompanying diagram, will make this clear. 
The principle is called the law of Avanzini. The 
smaller the angle made by the wind with the sail, 
the nearer the point of impact approaches to the 
front edge. This is true no less of the slightly curved 
surfaces of aeroplanes. As the angle of incline to 
the horizontal is more and more reduced, the wind 
acts at a point closer and closer to the front edge, 
and thus the aeroplane may, possibly, correct the 
dangerous tendency automatically, without the 
aviator having to make readjustments. 

To illustrate this principle I have made experi¬ 
ments with a catapult which, working within 
runners, threw pieces of cardboard horizontally, 



STABILITY 


27 

the cardboard being set so as to have an upward 
incline ; in fact as an aeroplane is set when it is 
travelling horizontally. This was managed by 
means of small wire carriers having various inclines, 
on which the cardboard rested, and which were 
themselves thrown with the cardboard. So far 
from pitching head downwards, the cardboard 
missiles would, even when the angle of deflection 
from the horizon was small, rise in the air, and some¬ 
times even turn over backwards, so strong was the 
action of the air on the front margin. 

In the bird’s wing there is a further automatic 
safeguard. It curves downwards at the back, 
at any rate that part of it that is nearer to the body. 
The wind acting over-strongly upon the downward- 
curving back part of the wing might, if the whole 
wing-surface were rigid, capsize the bird and send 
him diving head-foremost downward. But the 
elasticity of the feathers prevents an excessive lift. 
They yield to pressure, and the reduction of their 
curvature relieves the wing of any excess of pressure 
on its hinder part. 

The same principle holds good with regard to 
lateral stability. If a strong gust or eddy of air 
strikes the right wing while the left is struck with 
less violence, the feathers of the right wing yield 
to the rush of air and bend upward, so that the 
very force of the gust to some extent reduces its 
effect. Such elasticity would seem to be impossible 
in the case of an aeroplane without dangerously 
reducing its strength. And yet an aeroplane, from 
its enormous breadth, is far more liable than a bird to 
suffer from gusts falling with unequal force on its 


28 


THE FLIGHT OF BIRDS 


two extremities. At present the aviator is at a 
great disadvantage as compared with a bird. The 
great expanse of his planes is in itself a danger, 
and his machine has less power of automatic adjust¬ 
ment. Moreover, the most experienced pilot is 
a mere novice compared with a bird that has flown 
many times every day since he left the nest. Still, 
birds as flyers have already reached their zenith; 
for aviators greater things are still possible. 

Leaving the possibilities or impossibilities of the 
future, I must return to the bird as he is, and call 
attention to another point which promotes stability. 
The extremities of the wings of the best flyers 
have very little downward curve. If they are 
travelling very fast through the air, the primary 
wing-feathers may even bend slightly upwards, 
and aviators have found that the upward incline 
of their “ planes ” reduces the amount of rolling. 
Thus inclined they give less support than if curved 
downwards at the extremities, but the very fact 
that the air is allowed to escape easily to right and 
left is favourable to equilibrium. If an attempt 
is made by bending the extremities downward to 
check its escape, to coop it up, it may make a 
sudden rush from the right or from the left con¬ 
cavity and cause risk of a capsize. The wing, 
amply concave as it is till it begins to taper, and 
shallowing as it tapers, keeps in view the questions 
of lift and equilibrium; it holds the air, yet provides 
for its escape. Sometimes when a bird wishes to 
glide downward he will point his wings steeply 
upward—a method often adopted by pigeons— 
much more steeply than the right and left surfaces 


STABILITY 29 

of a monoplane. This attitude must tend to 
ensure a comfortable equilibrium (PL in, b). 

The question of the escape of air from beneath 
the curved surfaces seems to be of the utmost 
importance. Mr. Pilcher’s celebrated glider was 
punctured with many small holes which were 
intended to encourage a steady escape instead of a 
sudden upsetting rush on one side. I cannot judge 
whether this was a good plan for dealing with the 
difficulty. At any rate, the wing of even a large bird 
presents too small a surface to be treated in this 
way. But it is probable that the notches between 
the feathers at the wing’s posterior margin tend to 
prevent irregular escapes of air from below. The 
great soaring birds, whose steadiness as they circle 
with motionless wings is so marvellous, have the 
great flight-feathers parted like outspread fingers. 
Moreover, the force of the wind sometimes bends 
them conspicuously upward, thus giving them an 
incline that is recognized as favourable to equilibrium. 

Voluntary Adjustments. 

It cannot be gainsaid that there is much in the 
bird’s build that makes for automatic stability. 
Nevertheless, since the wind, at low levels at any 
rate, is a chartered libertine, full of capricious un¬ 
expected eddies, such automatic adjustments are 
altogether inadequate. The bird must be ready at 
a moment’s notice to give his mind to the question 
of balance, and make conscious voluntary adjust¬ 
ments. But even we ourselves find that movements 
we are perpetually making tend, through force of 
habit, to become automatic, and that in some cases 


30 


THE FLIGHT OF BIRDS 


we are not conscious of any difference between the 
voluntary and the involuntary. In the case of a 
bird the line is still harder to draw, not only because 
we cannot get within his mind, but because the 
movements which we are bound to label voluntary, 
since they cannot be mere unconscious reflexes, are 
so largely instinctive. When the young Swallow 
takes the first plunge from the parental nest and 
trusts himself to the air, he finds at once that he 
can fly ; the power of flight is instinctive. A child 
has to learn by much practice to co-ordinate his 
muscles ; no other young creature is so devoid of 
instinctive skill. The young Swallow, though he 
can co-ordinate his muscular activities well enough 
to fly with some success, has, of course, much to 
learn. At the outset he misses most of the flies and 
gnats, and his parents have to come alongside and 
put their captures in his mouth. But he seems by 
instinct to spread his tail when it should be spread, 
and, no doubt, though it is hard to see this, he takes 
a harder stroke with one wing than the other, when 
a harder stroke is required. For fore-and-aft balance 
he depends largely on his tail. He has not a long 
neck that he can bend or straighten out, and his legs 
are so short and light that no movement of them can 
have much effect. For lateral stability he must, as 
far as voluntary adjustments are needed, depend 
mainly on unequal wing-strokes.* That birds do 
take unequal strokes has been clearly proved by the 
camera in the case of Pigeons, Owls, Gannets and 
some others (.see PI. n). This inequality, sometimes 
so conspicuous in a photograph, the human eye has 
* See Chap. V. 


PLATE II. 



Right and left wings giving different strokes. A and B : Pigeons. 
C : Young Gull ; the right wing only is flexed at the wrist. (See 

Chap. II.) 


To face p. 30.] 




STABILITY 


31 


great difficulty in detecting, and but for the camera 
we should have had to own that, though the bird 
could hardly dispense with this method of balancing 
and steering, we could get no absolutely conclu¬ 
sive optical evidence. For fore-and-aft balance the 
means employed vary very much according to the 
build of the particular bird. Long-necked birds can 
move their centre of gravity forward and backward 
by extending or bending their necks. Ducks and 
Geese usually carry theirs stretched out, Herons 
habitually make a crook in theirs. Long necks 
are usually correlated with long legs, and long legs 
are no less serviceable in the matter of balance. 
This is shown by the fact that the long-legged have 
usually small, short tails, the long legs to some extent 
taking the place of the tail as balancers. A Flamingo, 
to take a conspicuous example, having legs of 
enormous length and a neck to match, has no need 
of much tail to regulate his fore-and-aft balance. A 
very little extension or retraction of legs or neck 
will set matters right if they are going wrong. But 
not only do long legs make a great expanse of tail 
unnecessary for balancing purposes, they must in¬ 
evitably hamper its movements if it is to be pulled 
downward with a view to checking speed or steering 
to right or left. It may well be that this necessary 
inefficiency of tail is in part the cause of the compara¬ 
tively clumsy steering of these big, heavy-legged 
birds. 

It can be shown, too, that for purposes of balance 
webbed feet very probably play a part of some 
importance. Not only are they fairly heavy, but 
they can be used as the tail is used, though not so 


32 


THE FLIGHT OF BIRDS 


effectively. The following table brings out very 
clearly the poverty of the long-legged and the web¬ 
footed in point of tail. I give first the actual length 
of tail, then the approximate weight of the bird. 
In two cases I have had to take the weight from a 
learned paper by two German ornithologists, and I 
suspect that their hen Sparrow-Hawk was far from 
plump. Lastly I have given the length of tail which 
each of the birds has for one pound of its weight. 
The results are very striking. Were I able to 
give the area of the tails, they would be much 


Bird. 

Length 
of tail. 

Weight 
of bird. 

Length 
of tail 
for each 
lb. 

weight 
of bird. 

Notes. 

Sparrow-Hawk (hen) 

Ins. 

8 

5*3 oz. 

Ins. 
24* 1 

Tail very 

Sparrow-Hawk (cock) 

6*5 

(Legal & 
Reichel)* 

5’2 oz. 

20 

broad. 

Lapwing 

4-1 

6-7 oz. 

9-8 

_ 

Wood-Pigeon 

6 

(Legal & 
Reichel)* 

1 lb. 4-25 oz. 

4-7 

Tail very 

Common Wild-Duck 

4-75 

lib. 15-7 oz. 

2*3 

broad. 

Tail 

Common Heron 

6*5 

3 lb. 0 • 6 oz. 

2-1 

pointed, 
small in 
area, weak. 

Curlew 

4*62 

2 lb. 4 oz. 

2*05 

— 


* Jahresbericht der Schlesischen Gesellschaft , 1879. 









STABILITY 


33 


more so. But the area varies so much, according 
as they are expanded or not, that I have found 
it very difficult to give measurements. However, 
we can bear in mind that the tail of the Sparrow- 
Hawk, for example, is not only very long but very 
broad, whereas the tail of the Duck is not only short, 
but narrow and weak, its inefficiency being even 
more marked than its small size. 

It is noteworthy that the bird which stands 
at the head of the list as being remarkable for size 
of tail is not a particularly short-legged bird. Two 
cock Sparrow-Hawks, of which I took measurements, 
had each legs 7 inches long (the toes being included). 
However, the legs are very thin, as if not intended 
for standing, but, armed as they are with long, 
efficient claws, for seizing a victim. It is often 
maintained that Hawks hang their legs down during 
flight, but this is certainly not usually the case. 
However, they could on occasion be lowered, to give 
the tail freer play, without much affecting the bird’s 
equilibrium. 

The main function of the tail is to prevent loss 
of equilibrium, and when large it plays its part 
wonderfully well. In spite of its great expanse, 
its weight is a negligible quantity and its working 
is splendidly prompt. Evolution has done wonders 
in thus metamorphosing the long, heavy tail of the 
bird’s reptilian ancestor. A number of vertebrae 
have been compressed to form the pygostyle, the 
small bony base of this wonderful piece of machinery. 
What weight there is is almost all accounted for 
by this little bone and the muscles that move the 
large spread of feathers. The muscles have the 


34 


THE FLIGHT OF BIRDS 


tail completely under command. At a moment’s 
notice they spread it, make it concave, lower it or 
raise it, lower this side or that. If the tail is spread 
and lowered, at once the hinder part of the body 
is lifted (see Pis. in and ix). The opposite effect 
will follow from the raising of the tail, so that it 
may catch the wind but little. If a Lark be watched 
through a field-glass as he rises facing a fair breeze, 
he will be seen to be perpetually busy with the work 
of correcting his fore-and-aft balance ; and it is to 
his tail that he trusts. A Chaffinch, perched on a 
rail with a fairly high wind blowing in his face, 
keeps his tail perpetually at work. The tail is, in 
fact, a fine balancer, even more important in this 
capacity than it is as a rudder. Were it not for the 
perfection of this balancer, a small bird could hardly 
land without danger to his eyes in the chevaux de 
frise of a furze bush. 

There are other balancing movements still to be 
considered. From our present point of view, the 
way the wings are held is of great importance. I 
have already pointed out that when a boat is 
tacking (see fig. 4, p. 8) it is the front part of the 
sail that does most of the work, and that similarly 
when the gliding bird inclines his wings at a small 
angle to the horizon, it is the front margin that 
gives him most support. If, then, he holds his wings 
fully expanded, so as to have as wide-stretching 
a front as possible, not only will the lift be greater 
but the centre of pressure (the point at which we 
may consider the force of the wind as being, so to 
speak, focussed) will move forward, and this will 
tend to give the bird’s body an upward incline. 


STABILITY 35 

And so, if he wishes to maintain his level or rise, 
he must extend his wings to the utmost; if, on 
the other hand, he wishes to descend rapidly, a 
partial flexing is the measure to adopt. To the 
aviator such a method is only to a very limited 




Fig. 14. 

House-Martin, gliding with wings partly flexed. 


extent possible ; he cannot half-flex his planes. 
Nevertheless, he is able to slide down a steep incline, 
much as a bird does. By means of his fore-and- 
aft balancing apparatus, he sets his aeroplane 
at the proper angle and shuts off steam. If he is 



36 


THE FLIGHT OF BIRDS 


a skilled and experienced pilot, he is, apparently, 
as steady as he rushes down towards the earth as 
a bird could be. But, if he did lose his balance, it 
would be fatal, whereas to the bird a momentary 
loss of equilibrium is of no consequence. 

Leaving aviators and their splendid achievements, 
I must describe another attitude sometimes adopted 
by a bird in gliding downward. He will extend his 
wings to the full, but hold them slanting upward 
[see PI. in). Obviously in this position the wings 
give less support, and so he descends. But he 
descends slowly, not with the rush that is character¬ 
istic of the head-foremost downward glide. The 
wings do not travel edgeways through the air and 
so they check his pace. Their upward slant is, as 
I have before remarked, advantageous to balance. 

I must now conclude this brief investigation of 
the bird’s stability when on the wing. What we 
see in the flight of birds—I am not now speaking 
of soaring—is not a steady, careful maintenance 
of equilibrium, but an instantaneous recovery of 
balance whenever it is lost. The bird can afford 
to be indifferent to the difficult problems which 
this subject presents. He has something much 
better than the power of maintaining equilibrium. 
However the gusts and vagaries of the wind may 
upset him, he can right himself at once. He owes 
his wonderful stability to some extent to his fine 
build and the elasticity of his feathers, but mainly 
to manoeuvres and adjustments that cannot be 
mere reflexes. The flying machine which he pilots 
is admirably built: still it can never dispense with 
a pilot. But his voluntary adjustments are largely 


PLATE III. 



A : Pigeon using tail for maintenance of equilibrium. (See Chap. II. 
and Plates IV., V., VI. and IX.). B : Pigeon gliding. C : Gulls gliding : 
(See Chap. II. and Chap. III.) 


To face p. 36.] 




STABILITY 


37 


instinctive, and even the niceties of adjustment 
that he has to learn must become through habit 
almost automatic. And the result of it all—of his 
fine build, his instinct, and his art—is the perfect 
stability of the machine; a thing to make the most 
skilful aviator envious : for who ever heard of a 
bird losing his balance and falling to the ground ? 
There are, of course, trials to which he may prove 
unequal. He may lose his way in a cloud, become 
exhausted when crossing a wide stretch of sea, or 
even fly stupidly into a telegraph wire. But an 
accident from want of balance would be evidence 
of morbid condition. 


CHAPTER III. 


MOTIVE POWER. 

LEVERAGE—PROPULSION—PHASES OF THE WING-STROKE. 

In describing gliding we have taken the motive 
power for granted, assuming that the bird already 
has momentum. We must now investigate his 
method of lifting and propelling himself. The lifting 
is the hard work, for the bird is so shaped that when 
he sets himself at a suitable angle the air offers but 
little resistance to his movement onward in a hori¬ 
zontal direction. To lift himself he must put great 
force into the downward beat of his wings, making 
their extremities move with such velocity that the 
air, the resistance of which increases as the square 
of the velocity, will very soon offer effective support. 
They will, in fact, become levers, each having its 
fulcrum mainly near the extremity, the weight to 
be lifted being, of course, the bird’s body, or, more 
correctly, the whole bird. The power is applied 
quite close to the body (see fig. 21, Chap. vn). The 
depressor muscle, the great muscle that springs from 
the breastbone and covers it with its great expanse, 
attaches to the humerus (or upper-arm bone) close 
to the nearer end. With a lever like this there is 
no economy of power—very far from it. What is 
gained is rapidity of movement. A quick movement 


MOTIVE POWER 


39 


of the near part of the wing will produce a movement 
of astonishing rapidity towards the further end, near 
the tips of the great primary-feathers. 

If x (see the figure) moves half an inch, x will 
move two inches. Preparatory to the down-stroke, 
the wing is lifted till it points straight upward, its 
anterior margin being turned in the direction of the 
bird’s flight. During the first part of its descent it 



Diagram to show the velocity with which the extremity 
of the wing moves. 

cuts edgeways through the air. But soon it turns 
face downwards and, the air opposing its descent, 
it finds a fulcrum. But, of course, there is some give. 
The fulcrum that the air supplies to the wing is, like 
the fulcrum that the oar finds in water, an imperfect 
one. When, however, the bird is taking full-length 
strokes, the wings appear to move with a far longer 
sweep than is really the case. With each stroke the 




40 THE FLIGHT OF BIRDS 

body rises, and this means a relative lowering of 
the wings. 

It is a wonderful thing that the air can supply a 
tolerably firm support, something that will do duty 
as a fixed point. Archimedes undertook to lift 
anything if he could find a fulcrum for his lever. 
The bird finds a fulcrum where Archimedes would 
not have thought it worth while to look for one. 
The bird when flying is, in fact, taking a number 
of jumps. Often when he appears to be travelling 
in a horizontal line he is really, as Professor Marey 
has shown in his wonderful photographs that give 
successive phases of the process, rising and sinking 
with each down-stroke and up-stroke. Thus the 
bird’s apparently horizontal line of progress is often 
an undulating one. But when he is travelling with 
great velocity, then there is no drop between the 
strokes; of this Professor Marey has obtained 
evidence. But there is probably some reduction 
of pace. Even though there is no rise with each 
stroke, the bird is nevertheless taking a series of 
jumps. And the marvel of these jumps, with no 
better take-off than the air, no amount of thinking 
can do away with. A man who is accounted a good 
high jumper can do very little if he has a poor take¬ 
off—if the ground is spongy. We all find it very 
hard work walking over soft snow when at each step 
we sink up to our knees before we find anything 
firm and resistant beneath our feet. We walk 
slowly and with labour along a beach where the 
small pebbles let our feet sink in. We climb with 
effort up a volcanic cone where, each step that we 
take, the small rounded ashes let us slide downward 


PLATE IV. 



Phases of the wing-stroke : photographs of Pigeons. A : Ready for the 
down-stroke. The series is continued on Plates V. and VI. (See 

Chap. III.) 


To face p. 40. ] 

















MOTIVE POWER 


41 


till we lose almost all that we have gained. But the 
bird has to deal with a material that seems far more 
shifty and undependable than small pebbles or 
volcanic ashes, or than snow at its worst. He over¬ 
comes the difficulty by means of levers calculated 
to give the utmost rapidity of movement. The 
muscular effort required is great, but his muscles 
are strong, and it is long before they tire. 

Propulsion.—Phases of the Wing-stroke. 

But the bird has not only to lift himself, or to 
maintain the altitude he has already gained. He 
must also have onward momentum. Were this 
wanting, he could not even lift himself, for air has 
little or no supporting power when it has just 
been disturbed. He must, therefore, be perpetually 
advancing to fresh columns of air that have not yet 
been shattered by the beating of his wings. I have 
already pointed out that, when birds are flying in 
flocks, each takes care to keep clear of the backwash 
of the bird in front of him—takes care to avoid tracts 
of air that have already been disturbed ; that a 
Pigeon, when he has a string tied to his leg, cannot 
maintain himself in air, however wildly he may ply 
his wings, when he has reached the end of his tether ; 
that when a Kestrel hovers without advancing there 
is always a breeze, so that each wing-beat descends 
on fresh, unbattered air. For ordinary flight the 
wings must be so adjusted as to propel as well as 
lift. This the bird can effect only if the front part 
of the wing is lower than the hinder part. Thus the 
parallelogram of forces comes once more to his aid. 

Let f b (fig. 16) represent a section through the 


42 


THE FLIGHT OF BIRDS 


bird’s wings, r being the front and b the back margin. 
The resistant air (w in the figure) will be equivalent 
to a wind blowing vertically upward. It will act 
at right angles to the plane of the wing, and the line 
representing its action will point not only upward 
but forward. During horizontal flight the front 
edge of the wing is slightly at a lower level than the 
back. But when the bird is rising and taking very 



Diagram showing the effect of the lowering of the front margin 
of the wing. 

energetic strokes, then the required incline is not 
obtained by that method only. The wing moves 
forward as it descends, so that at the end of the 
down-stroke, instead of making a right angle with 
the body, it points more forward than outward. 
When the wing is in this position, there is the down¬ 
ward slope that is wanted, from its base at the 
shoulder to the tip. One method passes gradually 
into the other ; indeed, the wing is inevitably shoved 
forward when the air lifts its hinder margin. On 



PLATE V. 



Phases of the wing-stroke, continued from Plate IV. A : Last phase 
of the down stroke. B : Beginning of the up stroke. C : Feathers 
bent upward during the up-stroke (see Marey, Vol des Oiseaux, 
p. 268, and Plate at end). D : Up-stroke continued. Continuation 
of series on Plate VI. (See Chap. III.) 


To face p ’42.] 







MOTIVE POWER 


43 


the forward-downward movement as giving the 
required incline I wish to lay stress, because treatises 
on flight do not, as far as I know, recognize this way 
of obtaining the downward slope, but speak only of 
the depressing of the front margin relatively to the 
back (see Pis. iv and v). 



Fig. 17. 

Gulls flying (after Marey). A.—25 photographs per second. 
B.—50 per second. 


I am now going to describe more in detail the 
different phases of the stroke. For this I must 
depend to some extent on Professor Marey’s photo¬ 
graphs. Other photographers, myself among them, 
taking snapshots, have caught the various wing 
positions. He shows us the various phases in series : 
they follow one another at the rate of 25 or of 50 
per second (see fig. 17 and Pis. iv, v, vi, vn). 



44 


THE FLIGHT OF BIRDS 


I will first give some account of a long and com¬ 
plete stroke. Preparatory to the down-stroke, the 
wing is raised till it points vertically upward, its front 
margin being turned in the direction of the bird’s 
flight. There may then be a moment’s pause, the 
wing, as it were, resting before it strikes its blow. In 
the case of gulls the next move seems to be a slight 
bend at the wrist-joint. After this begins the serious 
work. The wing descends with lightning speed, so 
fast indeed that this early phase of the down-stroke 
does not always appear in the series when photo¬ 
graphy has succeeded in depicting all the other 
phases. The great rapidity at this stage seems to 
indicate that it is not till the wing is approaching the 
horizontal that it begins to feel the resistance of the 
air and do its work of lifting and propelling. When 
this work is going on, the upward bending of the 
primary-feathers leaves us in no doubt about the 
fact. As the wing descends it points more and more 
forward. The way in which this is brought about 
is highly interesting. The big muscle which lowers 
the wing attaches to the front part of the lower face 
of the humerus (upper-arm bone) (see Chap, vn, 
fig. 21). Its pull, therefore, tends to lower the front 
of the wing relatively to the hinder part by rotating 
the bone. But the air, acting on the feathers that 
spread out rearward, greatly aids the muscle, lifts 
the hinder part of the wing, and encourages the rota¬ 
tion. But the work of the air does not end here. 
As soon as the wing has an upward incline from front 
to back, it cannot but move forward ; the mere 
action of the air on a surface so inclined cannot but 
bring this about. Thus the upward incline from 





PLATE VI. 



Phases of the wing-stroke, continued from Plate V. A : Nearly the 
same as Plate V., and from a different point of view. B, C, D : Up¬ 
stroke continued. (See Chap. III.) 


[To face p. 45. 




MOTIVE POWER 


45 


front to back, without which the bird would not 
make headway, is obtained first by the raising of 
the back part of the wing relatively to the front, 
and later, as the stroke advances, by the forward 
movement of the descending wing, which brings it 
about that the extremity occupies a position lower 
than and in advance of the base. For a moment 
let us consider the working of the wings in combi¬ 
nation. With the body they form a kind of funnel 
—obviously one side of the funnel is missing, but 
this is unimportant. Caught in this funnel and 
deflected from the wing-surfaces, the air impinges 
upon the body and lifts it. When the wing has 
strained forward and downward till it can strain 
no further, the muscles at length relax. The wing 
is no longer rigidly extended, but slightly bent at the 
elbow-joint, and soon at the wrist also. If the bird 
is rising and has little onward momentum, the 
Elevator muscle does the work of lifting. The great 
flight-feathers, which during the down-stroke have 
been pressed close against one another and so have 
made the wing impervious to air, are now slightly 
rotated, so that interspaces are left which allow the 
air to pass, and thus the raising is effected without 
much opposition (see Chap, vn, fig. 24). If, on 
the other hand, the bird has much way on, the air 
itself effects the lifting and little work on the part of 
the muscles is required. After the strain of the down- 
stroke, the Depressor muscle ceases its contraction, 
and, perhaps, the Elevator gets to work. In any 
case the front margin of the wing is no longer de¬ 
pressed relatively to the hinder margin, but is lifted. 
The wing lets the wind have its way, and is carried 


46 


THE FLIGHT OF BIRDS 


back, almost unresisting, but occasionally, as photo¬ 
graphs show, it resists enough to cause a backward 
bending of the flight-feathers. It looks as if the 
bird, when his wings are being lifted by the rush of 
air into position for a fresh stroke, checked them for 
a fraction of a second in order to save himself from 
loss of altitude. It is possible this check in the 
course of the up-stroke may be the normal thing, 
or it may be only occasional [see PI. v). When 
the wing is thus raised by the rush of air—more 
strictly by the resistance of the air to its momentum 
—the flight-feathers are pressed against one another 
and there are no gaps to make the lifting easy. But 
no such help is required. The wing is blown back 
as far as it has freedom to go, and at the end of its 
rearward movement it is no longer facing as it was ; 
its under-surface is facing outward and its anterior 
margin is looking towards the bird’s head. When 
the wing is in this position it is easy to raise it com¬ 
pletely and bring it forward; in fact it moves edge¬ 
ways. Anatomy supplies very remarkable evidence 
that the raising of the wing requires little effort. 
Whereas the muscle which lowers the wing is red, 
ridged, and granulated, the Elevator muscle is paler 
and exposes a smooth surface when it is cut. The 
former is by far the better class of muscle, capable 
of long, unflagging effort. 

In ordinary horizontal flight most birds take a 
much shorter stroke than the one I have just 
described, nor is the wing pointed much forward 
{see PI. vn). The upward incline from front to 
back that is needed is obtained by a slight lowering 
of the front margin relatively to the back, mainly 


PLATE VII. 



Gulls flying with short stroke. A : Lesser Black-backed Gulls. 
B : Black-headed Gulls ; the lower bird on the left with wings 
bent at the wrist, shows an early phase of the up stroke. (See 

Chap. III.) 


To face p. 46.] 








MOTIVE POWER 


47 


towards the extremity of the wing, and this gives 
propulsion enough. This short stroke is the one 
adopted by gulls in their ordinary leisurely flight. 
The wings give the air a sharp slap, and this with a 
bird so well built for flight and so skilled is very 
effective. It is to be noticed that, when gulls are 
taking short, leisurely strokes, the wings during the 
down-strokes are very distinctly curved from their 
base to their extremity. This curve prevents a too 
easy slipping away of the air and so increases the 
wing’s lifting power ; but when a stronger stroke 
is taken the primary-feathers bend upward. 

The question how a bird lifts and propels himself 
I have now briefly answered. In other chapters I 
shall try to describe the build of this living flying- 
machine, to my thinking the noblest of all craft that 
sail the air (see Chaps, vn, vm, and xi). 

Wr' 


CHAPTER IV. 


STARTING. 

THE PRELIMINARY JUMP—LOSS OF ALTITUDE BETWEEN WING- 

STROKES-THE WING’S FREEDOM TO ROTATE-AEROPLANES- 

MUSCLES-BIG BIRDS AND SMALL. 

The Preliminary Jump. 

I have shown how a bird when flying maintains 
and propels himself, when we have given him in 
imagination a good start. We have now to study 
his method of starting. 

Let us suppose that he is standing on the ground ; 
if he is to fly he must somehow get clear of it. For 
this purpose his long, strong legs are of great service 
to him ; it is owing to them that he is so good a 
starter. Such is the bird that I think of as typical. 
Not only are his legs strong, but they are long, 
unless we compare them with those of a Wader or 
a Heron, birds that for their particular mode of 
life have developed an inordinate length of leg. 
The Puffin and the Swift, by their helplessness on 
level ground, call attention to the remarkable leg- 
power of the ordinary small bird, or bird of moderate 
size. The Swift and the Puffin cannot take the 
preliminary jump with which flight must begin. 
Bulky, short-legged birds all rise with difficulty ; 
the jump that should in a moment obtain for them 
freedom of wing action is beyond their power. 






PLATE VIII. 



A and B : Pigeons starting to fly. B : A very hurried start. C Pigeon 
rising with body inclined steeply upward. D : Herring-Gulls starting ; 
photographed in September, when they were moulting. '{See Chap. TV.) 


[To face p. 49. 











STARTING 


49 


It is not that they are deficient in lifting power, 
for it is said that an Eagle can carry a weight as 
great as its own, and I have seen a Falcon flying 
off with a victim that was not far short of itself 
in point of size. 

Whether big or small, birds all fold their wings 
neatly upon their backs, where they cannot possibly 
interfere with freedom of leg action. In order 
to appreciate the excellence of this arrangement, 
we must compare the bird with the bat or with the 
pterodactyle, whose wings were remarkably bat-like. 
Extending, as they did, far back and attaching to 
his legs, they must have been as bad an encumbrance 
as long skirts. Though, no doubt, a fine flyer in 
a bat-like style when once launched on his way, 
the pterodactyle was a poor starter. The bird’s 
legs, on the contrary, are not sacrificed to his wings 
[see PI. viii). Unless he is one of the specially 
bad starters, he jumps lightly into the air and is off. 

With regard to the heavy, lumbering way in which 
many big birds rise, a great deal is to be learnt 
by watching a Heron start to fly from level ground. 
Unlike the Condor, the Comorant, the Puffin, and 
the Swift, he has no difficulty in getting under 
way. True, he does not rise with a steep incline 
as an ordinary small bird or a Pigeon can, but a 
gradual ascent he carries out without difficulty. 
He drops from the top of his long legs on to the 
spacious fields of air and is at once clear of the earth, 
with ample room for the plying of his wings. On 
the other hand, the big short-legged birds and 
small birds like the Swift, with very short, feeble 
legs, have no room for a full sweep of their wings, 


50 


THE FLIGHT OF BIRDS 


unless they have, by some means other than a 
jump, attained some elevation. Hence the difficulty 
they have in starting from level ground. But even 
when this full sweep is possible big birds are still 
only capable of ascending with a gentle incline, and 
this is a fact that we must try to account for. 

Loss of Altitude between Wing-Strokes. 

To my thinking the explanation is this : since 
the big bird necessarily takes a longer stroke and 
requires more time to raise his wings for a fresh 
downward beat, he must inevitably lose more 
altitude between wing-strokes than the small bird. 
Professor Marey gives the rate of stroke for a 
number of birds as registered by means of scientific 


apparatus.* 

Strokes per 
second. 

Sparrow .. .. .. 13 

Duck .. .. .. 9 

Pigeon ., .. .. 8 

Marsh-Harrier .. .. 5| 

Screech-Owl.. .. .. 5 

Buzzard .. .. .. 3 


In the case of a Sparrow it is evident that there 
is no time for a drop between the strokes, though 
compared with those of most insects even a 
Sparrow’s wings move slowly and heavily. The 
leisurely working of a Heron’s wings is familiar 
to everyone who ever takes the trouble to observe 
birds ; usually he flaps along with only 130 strokes 
a minute or even slightly less. The Marsh-Harrier 

* See his Vol des Oiseaux y p. 100. 






STARTING 


51 


has a much quicker beat, but even with such a rate 
as his a drop between the strokes is quite possible 
unless he has plenty of way on. And now we are 
getting to the explanation of the big bird’s method 
of rising. In order to avoid losing altitude between 
the strokes, he must take care that he has momentum, 
and if he is to have momentum he must be content 
to ascend by a gentle incline. 

The Wing’s Freedom to Rotate. 

Moreover, there is a want of freedom about his 
wing-movements which makes him incapable of 
anything but a very gradual ascent. If he were to 
incline his body steeply upward, after the manner of 
a small woodland bird that, making for a gap in the 
dense spreading boughs overhead, mounts almost 
vertically, he would have to rotate his wings in a 
way that is impossible for him ; he would have to 
lower the front margin relatively to the back, or 
else they would beat in such a way as to drive him 
backward instead of lifting him. In fact, he has 
too little freedom at the shoulder. He cannot 
set his wings as a steep ascent requires. The 
small bird’s wings, on the other hand, rotate so 
freely that even when he sets his body with a steep 
upward slant he can still turn them over so that 
they have an up-and-down beat and raise him sky¬ 
ward. 

But though, speaking generally, the small bird is 
capable of a steeper ascent than the big bird, yet it 
would be a great mistake to imagine that if we were 
to arrange birds according to their weights, from the 


52 


THE FLIGHT OF BIRDS 


lightest to the heaviest, we should at the same time 
be arranging them according to their angles of ascent 
—the angle which each makes with the horizon when 
the line of his ascent is as near to the vertical as his 
build and his powers allow. In the case of all very- 
big, bulky birds, I believe the wing rotates reluctantly 
and with difficulty at the shoulder. The Gannet, 
Pelican, Cormorant, Eagle I have tested, and found 
that they have very little power of lowering the front 
edge of the wing relatively to the back ; one Eagle 
was a partial exception. I have never had a live or 
a freshly-killed Condor at my disposal, but there is 
reason to believe that the Condor is among the 
stiffest of the stiff. The way in which Condors are 
trapped in Chile—it is described by Darwin in his 
Journal of Researches (Chap, ix)—supplies indirect 
evidence of the great bird’s limitations. The plan 
is to place a carcass “on a level piece of ground 
within an enclosure of sticks with an opening, and, 
when the Condors are gorged, to gallop up on horse¬ 
back to the entrance, and thus enclose them : for 
when this bird has not space to run it cannot give 
its body sufficient momentum to rise from the 
ground.” In fact a Condor cannot rotate his wings 
and set them as they must be set if his line of 
ascent is to make a large angle with the horizon. 
As far as I know, all birds of great bulk have this 
defect, but when we come to birds of medium size we 
find great variations, and it soon becomes apparent 
that it is largely a question of habitat and environ¬ 
ment. The Pheasant and Duck—I have tested the 
Mallard, the Sheld-duck and the Teal as representa¬ 
tive of the Ducks—have great freedom of movement 


STARTING 


53 


at the shoulder, and can, moreover, point their wings 
forward so that at the finish of the down-stroke they 
have an upward incline from tip to base. On the 
other hand, the Partridge and the Herring-Gull have 
very little power of rotation. The striking contrast 
between the Partridge and the Pheasant throws a 
great deal of light upon the question. The Pheasant 
is a denizen of woods and has often to make for an 
opening in the branches that shows itself almost 
directly over his head. Heading straight for it, he 
points his body almost vertically upward, but, in spite 
of that, his wings have an up-and-down beat, and turn 
the concavity of their nether side towards the ground 
and their upper convex surface towards the sky. 
The Partridge, on the other hand, frequents plains 
and open fields where there are no entanglements 
to make a nearly vertical ascent even an occasional 
necessity. A Wild Duck sometimes finds it expe¬ 
dient to mount upward from her nest among the 
bushes in the same style as the Pheasant just 
described, and I have seen a Wigeon, without any 
apparent advantage to himself, shoot up thus from a 
large piece of open water. All birds, if we except 
those that frequent only open water, bare cliffs, bare 
hills or unwooded plains, may find, any moment, that 
they have to make a rapid ascent up a steep incline ; 
life itself may depend upon it. Hence great freedom 
at the shoulder is very common. I have observed 
it not only in the Pheasant and the Duck, but in the 
Jackdaw, Crow, Raven, Chough, Jay, Magpie, and 
Quail. No wing, I think, rotates more freely than 
that of the domestic Pigeon. Were it not for this, 
the bird in PI. vm could not achieve, as he is 


54 THE FLIGHT OF BIRDS 

evidently doing, the feat of an almost vertical 

ascent. 

The big and bulky birds then, such as the Gannet 
and the Condor, having a much slower wing-stroke 
than the light-weights, must make it their first 

object to attain momentum, otherwise they will 

lose altitude between the strokes. When they 
have got way on, they can rise, but the line of their 
ascent is a gentle slope. For a steeper ascent the 
set of their wings unfits them. But there are birds 
of medium size, such as the Duck and the Pheasant, 
which, striking with very great rapidity and rotating 
their wings as freely as any small bird, are capable 
of raising themselves almost vertically through 
the air. 

Aeroplanes. 

In a country like England, with trees and hedges 
almost everywhere, an aeroplane capable of a 
steep ascent is a great desideratum. Captain 
Brooke-Popham, writing on military aviation, says 
that “ with no wind a fully-loaded machine, with 
observer, could get off a hard level field in a length 
of 120 yards and clear a fair hunting hedge at the 
end. Our Air Battalion “ Farman ” can do this 
in 90 to 100 yards without any difficulty.”* Thus 
even the “Farman” requires a great deal more 
space than a Condor or a Comorant, birds noted 
as slow, lumbering starters, and though aeroplanes 
are constantly being improved it may well be 
doubted whether a steep ascent will ever be 
achieved. 


* The Army Review , January, 1912, p. 89. 


STARTING 55 

Muscles. 

To rise in air before he has got up pace is hard 
work for any bird unless he can get the wind to 
help him, for a great strain is put upon the muscle 
that lifts the wing. And here I may call attention 
to the remarkable development of the Elevator 
muscle in the Pheasant. Its weight amounts to 
nearly one-third of that of the Depressor. It is 
very pale and has little lasting power, but for a 
brief effort it is very effective. Its development 
in the Duck is considerable, but not equal to what 
is found in the Pheasant. As soon as the bird 
begins to travel rapidly, there comes an easier time 
for the overtaxed Elevator, for the resistance of 
the air to the onward impetus is sufficient to lift 
the wings, and the impetus is due mainly to the 
work of the great Depressor muscle. Hence the 
Depressor not only lowers the wing, but indirectly 
lifts it. The Elevator is not fitted for long-sustained 
effort. The Depressor is a redder, rougher, more 
granulated muscle, and its different colour and 
texture are indicative of superior quality.* 

Big Birds and Small. 

As a rule, when a small bird flies, his line of 
flight is undulating. For him to rise is easy ; a 
few strong, rapid strokes lift him. He then partly 
flexes his wings and glides onward and slightly 
downward. This is very noticeable in the case of 
the Woodpecker. He flexes his wings more than 
most birds, and so the dipping character of his flight 


* For more on this subject, see p. 45 and Chap, vii : Muscles. 


56 THE FLIGHT OF BIRDS 

is more conspicuous. The big bird, on the other 
hand, dare not lose altitude in so reckless a way, 
since when he has once lost it he with difficulty 
recovers it. He ploughs steadily on, whereas the 
small bird with a few rapid strokes gains altitude, 
then, like a bicyclist utilising his free wheel, glides 
restfully and rapidly onward, not minding the loss 
of some of the height he had gained. His stroke 
is more rapid, but he is able to take frequent 
easies. * 

I have now compared and contrasted big birds 
and small, but even the biggest birds that fly are 
not very big ; as compared with the larger mammals 
they are diminutive. Why, among all the birds 
that fly, are there none that weigh even half as 
much as, for example, a Zebra ? 

Helmholtz, dealing with this subject, produced a 
formula which, backed as it was by the authority of 
a great man, was too readily accepted, regardless of 
the fact that it was not founded on data obtained 
by experiment. There is nothing so misleading 
as mathematics when the premises are unsound. 
Helmholtz started with undeniable facts. If a bird’s 
linear dimensions be multiplied by 4, then the area 
is multiplied by 16 (4 2 ), and the bulk, which must 
nearly correspond to the weight, by 64 (4 3 ). So far 
good. He showed that the weight increased more 
rapidly than the supporting area. But when he 
went on to maintain that the power required to lift 
the bird increased at a still more rapid rate—that, in 
the case I have taken, it would be 128 (4£)—then he 
was building a theory without a proper foundation 
* On big birds and small birds see Chap, i, pp. 18-22. 


STARTING 


57 

of proved fact. Even now, though the subject has 
been much studied, it would be very rash to venture 
on a formula. The big bird has great advantages. 
He can manage, as we have seen, with a relatively 
smaller expanse of wing, for the area, being greater 
absolutely, does not so readily allow the escape of 
air at the margins. Moreover, his wing, being longer 
absolutely, is a more powerful lever. The great 
weights which Eagles, for example, carry show that 
there is no deficiency of lifting power. But the big 
bird’s wings have not the easiness of rotation at the 
shoulder-joint which makes it possible for a Green¬ 
finch, for instance, or a Pheasant, to rise with a steep 
incline ; he cannot put himself in the right attitude. 
It does not appear, however, that bulk in itself is 
any handicap. 

If this be so, it may well be asked why even big 
birds are quite small when compared with the larger 
mammals. I have already pointed out that the big 
bird, if his legs are short, has difficulty in beginning 
a flight, and so lacks a very important accomplish¬ 
ment. Then why are they not all mounted on stilts, 
like the Flamingo ? But legs of such length, or even 
the half of it, would for many birds be most incon¬ 
venient appendages, for a diving bird most of all. 
They would not help him to rise from the water, and 
they would be clumsy things beneath the surface. 
And thus among big birds there are many that for 
purposes of flight are handicapped by shortness 
of leg. Diminutive size brings with it another 
advantage. The small bird comes more rapidly to 
maturity. In his second spring, when he is not yet 
twelve months old, the Blackbird has already paired 


58 


THE FLIGHT OF BIRDS 


and is busy nesting. The Gannet is known, I 
believe, not to nest till his fifth year ; the Eagle 
comes to maturity later, possibly not till his tenth 
year. In rate of reproduction, therefore, a most 
important matter for the species, the small bird has 
a great advantage. And thus we can account for 
the small size of birds compared with that of mam¬ 
mals without recourse to any such theory as that of 
Helmholtz. The small bird is a better starter than 
the big, and he comes to maturity more quickly. 


CHAPTER V. 


STEERING. 

A VARIETY OF METHODS-GOOD STEERERS AND BAD. 

A Variety of Methods. 

A bird can steer when his tail is gone. A Rook, 
when some accident has robbed him of this useful 
rudder and balancer, can still make shift. He is 
not like a ship left rudderless. It is evident, 
therefore, that the tail is not the bird’s sole steering 
apparatus. If he wishes to steer to the left his 
usual method is to fling himself on his left side, 
the left wing pointing downward and the right 
wing upward, the two being in line with one another, 
while his head is pointed in the direction in which 
he wishes to travel (see PI. ix). Then he can no 
longer progress along his former line of advance, 
for the expanse of his wings will check him. He 
travels, therefore, to the left, i.e. towards the point 
towards which his head is directed. But how does 
he effect the necessary change of balance ? There 
is no doubt that he occasionally gives a harder stroke 
with one wing than the other, a thing which the 
camera sometimes detects, though it is difficult 
for the eye to see it clearly. On Plate n are some 
photographs in which the wings have been caught 


60 


THE FLIGHT OF BIRDS 


at a moment when they were not held symmetrically. 
Another plan is to bend sideways at the waist, 
so as to move the centre of gravity towards one 
side. I was long doubtful whether this plan was 
actually adopted. At length I have obtained a 
photograph of a Gull, taken from below, where the 
waist, thus bent, is clearly shown, but unfortunately 
this photograph will not stand reproduction. Below 
I give some measurements which show that birds 
that are good steerers have greater suppleness of 
waist than clumsy steerers. 

The tail also is undoubtedly used for steering, 
though, I believe, it is more frequently of service 
in maintaining or restoring equilibrium. The rule 
is, as I have shown, that if the legs are long the 
tail is small, and as it is indisputable that long 
legs are very useful for balancing and of but little 
use in steering, we may infer that the tail is more 
a balancer than a rudder. Nevertheless, it is of 
much use in steering. Though its edges look to 
right and left, it can be made an effective rudder 
by the lowering of one side more than the other. 

Web-footed birds probably use their feet occa¬ 
sionally for steering purposes, but I doubt if their 
action counts for very much. When a Duck, for 
example, is hurtling through the air, if he lowers 
one foot, the resistance of the air will double it up, 
not expand it. It is very different when a Duck 
in swimming kicks backward; the water acts 
upon the under-surface and spreads the webs to 
their fullest extent. Some birds when alighting 
use their feet in the same way. A Gannet, for in¬ 
stance, kicks hard in order to correct his balance 


PLATE IX. 



A and B : Gulls steering by throwing themselves on one side. A : 
Steering to the right and looking to the left. C and D : Pigeons 
showing the tail used as a rudder. (See Chap. V.) 


To fare p. 60.] 
























































































STEERING 


61 


and get into the right attitude for settling. It is 
a clumsy performance. 

The movement of the head to one side or the 
other has, no doubt, some slight influence on the 
balance. But the skull is very light, a great part 
of it as thin as paper, and one may occasionally 
see a Gull look to the left while he steers to the right 
(PI. ix), just as a skater, who is quite at home 
on his skates, can make such minor movements 
without in any way upsetting his balance. This 
subject naturally brings to mind the wonderful 
way in which Mr. Cody showed the lateral stability 
of his biplane. He carried a passenger who stood 
at a distance of ten and a half feet from the centre 
where he himself was piloting. When a bird 
turns his head, it is far more probable that he is 
directing his eyes, to right or left, towards some 
object that has attracted his attention, than that 
he is regulating his balance. Mr. Bentley Beetham 
has a capital photograph* in which he has caught 
five Gulls in the act of turning their heads to the 
right. This may possibly be due, as he suggests, 
to a local current of air with which they all equally 
have to cope. But it seems far more likely that 
they have all suddenly caught sight of some object 
of interest, a fish or shoal of fish, at no great 
distance. What would one not give for a photograph 
of a large flock of birds scudding through the air 
high aloft—for a photograph taken at the moment 
when by some common impulse, as if following some 
leader, they each and all change their course ? Might 
we not see, if we could obtain such a photograph, 

* British Birds, Dec., 1910. 


62 


THE FLIGHT OF BIRDS 


all the heads turned one way, the object being to 
follow with the eye the movements of the leader or 
leaders of the flock ? 


Good Steerers and Bad. 


There are good steerers and bad steerers. 
Contrast the Swallow, the Lapwing, or the Sparrow- 
Hawk, those adepts at sudden swerving and 
doubling, with the Duck! The Duck becomes the 
slave of his own ponderous momentum, and changes 
his course slowly and with effort. The Swallow, 
the Lapwing and the Sparrow-Hawk have all of 
them a fine expanse of tail, while the Duck’s is small 
in area and, what there is of it, feeble. Efficiency 
or inefficiency of tail, no doubt, accounts for a 
great deal. But it is not the only factor to be 
considered. The Duck, though very strong, is 
lacking in agility; the three other birds have far 
more suppleness, I believe, notably at one im¬ 
portant point. Though birds’ backbones are, 
below the neck, very stiff, yet they allow of a good 
deal of bend, either up and down or sideways, at the 
waist. I have measured the amount of sideways 
bend in some few species and found that, as far as 
my far from complete evidence goes, good steerers 


Angle formed by backbone 
bending sideways at waist. 


Kestrel 

Swallow 

Swift 

Common Tern 

Kestrel (another specimen) 

Domestic Duck 


141*5° 

150° 

153-5° 

155° 

156° 

165° 




STEERING 


63 


have supple waists and poor steerers comparatively 
stiff ones, the Duck being a good deal the stiffest 
of those I have examined. 

It is difficult to account for the difference between 
the two Kestrels, but there is no doubt that the 
Duck is, of the birds in question, decidedly the 
stiffest. In fact these measurements make it a priori 
probable that suppleness of waist counts for 
a good deal. The result of the sideways bend 
must be that the bird is thrown on to its side—the 
attitude which is assumed when a sudden turn is 
to be made. 

For steering purposes, then, a bird has various 
methods at his disposal. He can take unequal 
wing-strokes or bend at the waist. Either of these 
means will put him in the attitude in which we 
see him steering to right or left, one wing pointing 
downward, the other upward. And he can use 
his tail as a rudder. Some birds may, no doubt, 
steer partly by means of their feet, but I doubt 
whether a foot is a very effective rudder. The 
Duck’s webbed feet do not seem to make him an 
adept at turning. 


CHAPTER VI. 


STOPPING AND ALIGHTING. 

A bird alights without any jar. Let us imagine 
that when we first catch sight of him he is flying at 
some height above the ground. Wishing to descend, 
he will give his wings an upward slope and float down 
in the style usually preferred by Pigeons, or more 
probably he will slant his body, from tail to head, 
downwards and, partly flexing his wings, glide rapidly 
towards the earth. On nearing his landing place he 
will suddenly let his hind-quarters sink, and give 
his body an upward incline ; his wings, spread wide, 
present their whole under-surface to the air. This 
soon checks his momentum. But if he wants to 
make a very sudden stop he gives a stroke with his 
wings, and this, when the body is nearly upright, 
as a glance at one of the photographs (see PI. x) 
that illustrate the process will show, must bring him 
at once to a standstill. And just after landing, 
perhaps (notably if he is a Tern, a Lapwing or a 
Pigeon) he will raise his wings high above his head 
as if to stretch and refresh the muscles, in which 
attitude he looks very beautiful. A beautiful thing, 
too, is the folding of the wings ; it is all so quickly 
and so neatly done. A Tern or even a Pigeon when 
he alights has all the grace of the “ Herald Mercury, 
new lighted on a heaven-kissing hill.” 


PLATE X. 



A and B : Pigeons alighting. Though in B the feet are 
still clear of the window-ledge, I think the bird is giving 
his wings the final stretch that with many species is pre¬ 
liminary to the folding of them. C : Pigeon checking speed 
before alighting. {See Chap. VI.) 


To face p. 64.] 










STOPPING AND ALIGHTING 


65 


As birds are so careful in alighting to avoid all jar, 
it is remarkable that some species have habits which 
would seem likely to cause concussion of the brain. 
It is astonishing that the Nuthatch, for example, 
when he has been half the day hammering at nuts 
with tremendous vigour, yet suffers no bad conse¬ 
quences. But we must bear in mind that the bird’s 
neck, with its peculiar saddle-and-rider vertebrae, 
is more supple than a snake, whereas the backbone, 
except just at the waist, is remarkable for its rigidity. 
Hence, probably, the care and dexterity in alighting 
that contrast so strikingly with the reckless use of 
the beak and head as a hammer by the Nuthatch, 
the Woodpecker, and other birds. It must be owned 
that there are some birds which, when they are 
alighting, are very unlike the “Herald Mercury.” 
Among these we must count the Gannet. When he 
gets near his nest upon some cliff, he paddles hard 
with his legs, and at last settles clumsily down. Mr. 
Bentley Beetham (British Birds , May, 1911) has 
some very good illustrations of the Gannet’s style 
of alighting. One of them shows a bird that has so 
lost command of his movements, that he is flopping 
most ungracefully onto his nest. His breast is 
resting on the pile of dry seaweed, his outspread 
wings on the rock at either side. It is an astonishing 
attitude, but it must be borne in mind that the 
Gannet is in the habit of taking headers from a great 
height, and that his breast is shielded by air cavities 
beneath the skin, first-rate air-cushions that greatly 
reduce the shock when he dashes into the water. 
This may account for his descending breast foremost 
onto his nest. But the Cormorant, who has no 


66 


THE FLIGHT OF BIRDS 


air-cushions, though he is the Gannet’s near relative, 
is also clumsy in alighting, and Mr. H. F. Wither by 
has described to me how Cormorants, wishing to 
alight upon a post, will sometimes make a bad shot, 
pass it, and have to turn and try again. A Gull will 
alight gracefully enough on the water, but when he 
is aiming at a particular spot on a rock, for example, 
he will often paddle as awkwardly as a Gannet. The 
Lapwing is in a very different category ; he is noted 
for his power of suddenly stopping, of making every 
possible turn. When you get near his nest he has a 
way of flying at you in threatening style, then 
suddenly checking himself and making off. He is a 
perfect master of bluff. A pair of Lapwings, by such 
menacing swoops and turns, will drive off a Crow 
whom they suspect of looking for their nest. In 
order to stop suddenly, what is wanted is a great 
expanse of feather, and I cannot help associating 
the Lapwing’s remarkable wings—they are so 
strikingly broad at the extremity—with his well- 
known shock tactics. 

I have already remarked on the skill which small 
birds show in alighting on their perch. Were a 
Linnet not very clever at making all the required 
adjustments, she would seldom reach her nest, some¬ 
where deep in a gorse bush, without accident. 


CHAPTER VII. 

THE MACHINERY OF FLIGHT. 


THE BREASTBONE AND THE CONNECTED BONES-MUSCLES AND 

QUALITY OF MUSCLE-THE SCAFFOLDING OF THE WING- 

PNEUMATIC BONES-STIFFNESS OF WING-EXPANSE OF BONE- 

THE SPREADING OF THE WING—STRUCTURE OF A FLIGHT- 
FEATHER—MOULTING—LEGS. 

When we carve a Partridge or a Grouse, since other 
questions seem at the time of more pressing interest, 
we seldom give a thought to the fact that we are 
slicing and disjointing what has been a most mar¬ 
vellous flying machine. Yet so it is. When we 
carve the breast, we first cut through the Great 
Pectoral, the powerful muscle that lowers the wing. 
Lying below it we find, easily distinguishable, a 
much smaller, lighter-coloured muscle — in the 
Grouse and its kin markedly lighter. This smaller 
muscle lies in the angle between the keel and the 
sternum (or breastbone) proper; its work is the 
lifting of the wing. 

The Breastbone and the Connected Bones. 

The framework, the bony skeleton, is wonderfully 
adapted to the purpose to which it is put. It is 
most important, to begin with, that there should 
be a wide expanse of bone from which the great 
flight muscles may spring. And so the area of the 
sternum is increased by the large projecting keel. 


68 


THE FLIGHT OF BIRDS. 


The Ostrich has no keel, and is incapable of flight. 
The Hoatzin, that curious South American bird, 
according to all observers a most feeble flyer, has 
the front part of the keel missing. In every case 
the shape of the keel has its significance and is well 
worth observing, since it indicates the style of flight. 
The Duck and the Guillemot, having very long 
breastbones and keels, have of course very long 
flight muscles and a long muscle is capable of more 




Fig. 18. 

Breastbones of (1) Guillemot—actual length of keel inches. 
(2) Falcon. Drawn to scale. Compare that of the Adjutant, 
a Stork famed for his soaring. 

contraction than a short one. A glance at their 
breastbones, therefore, tells us that they are birds 
that fly with a long stroke. A Guillemot’s wings are 
so small that a long, strong stroke is a necessity 
for him. A Duck, during horizontal flight, does 
not raise his wings very high, but the stroke is pro¬ 
longed till the wings can strain no further downward. 


69 


THE MACHINERY OF FLIGHT 

The Tern and the Falcon, on the other hand, have 
short, deep keels, and therefore short but big and 
strong flight muscles. Accordingly, their ordinary 
stroke is short but powerful. It is highly important 
also that there should be a firm pivot on which the 
wing may rest as it moves. During part of the 
wing’s down-stroke there must be considerable 



Breastbone and connected bones of Adjutant—actual length of 
keel 4f inches. Cl. : Clavicle ; Co. : Coracoid ; Sc. : Scapula. 


pressure inward, and were the skeleton to give, 
were it to supply only an unsteady wobbling pivot, 
vigorous flight would be out of the question. How 
is the pivot formed ? There is a strong bone, the 
coracoid, which springs from the front part of the 
breastbone and points forward and outward. It 




70 


THE FLIGHT OF BIRDS 


slopes more outward in strong than in weak flyers, 
and, as a rule, more in big birds than in small. At 
the shoulder-joint the coracoid is met by two other 
bones, the clavicle or merrythought (which varies 
much in make and strength), and the scapula or 
shoulder-blade, and the three bones together form a 
first-rate pivot, so that the strong wing-beats do not 
shatter the bird’s framework. 



Fig. 20. 

Clavicle of (1) Tern ; (2) Eagle. Drawn to scale. 

Muscles and Quality of Muscle. 

From the keel and from the outer part of the 
sternum (thus covering up the small Elevator 
muscle), and also from the clavicle, springs the Great 
Pectoral—the Depressor muscle ; it attaches by a 
short tendon to the front part of the under-surface 
of the humerus (upper-arm bone), and, attaching 
where it does, its pull tends to lower the front part 
of the wing relatively to the back. And springing 
as it does not only from the breastbone but also from 
the clavicle, it pulls the wing, not downward and 
backward, but simply downward. The underlying 
Elevator sends out a long tendon that passes 




THE MACHINERY OF FLIGHT 


71 


through a little tunnel formed by the three bones, 
which meet at the shoulder-joint, and after passing 
through the tunnel attaches to the upper side of the 
humerus, near to its preaxial or front edge. Its 
action is to erect the wing upon its pivot. Attaching 


D 



Fig. 21. 


Humerus of right wing of Eagle. Actual length 6f inches. D : 
Flat area near the front margin of the under-surface, where the 
tendon from the Depressor muscle attaches. 



where it does, it must necessarily bring the wing 
into the proper position for beginning the next stroke 
with its front edge looking in the direction of flight. 




72 


THE FLIGHT OF BIRDS 


Sometimes, in order to carry out a movement, two 
muscles have to antagonise one another. When a 
bird wishes to check his speed suddenly, he lets his 
body hang downward, sometimes almost vertically, 
and holds his wings so that their under-surface faces 
in the direction of his flight. Now, if the Depressor 
were alone holding the wing, it would lower the front 
relatively to the back, and the resistance of the 
air would reinforce its action, making the wing turn 
its under-surface downward as in ordinary flight, 
whereas for checking speed it must face to the front. 
To orient it thus, by holding fast the front margin 
of the upper face of the humerus, is the work of 
the Elevator. The antagonising of one muscle by 
another is, of course, constantly going on. Were 
it not for this, with its steadying effect, no movement 
could be carried out with precision, no attitude could 
be maintained. There are, of course, other muscles 
which contribute to the working of the bird’s wing, 
and, notably, there is a third pectoral, the office of 
which is to draw the wing back. But it is no part 
of this short treatise to describe the work of minor 
muscles. 

I have already alluded to the different qualities 
of muscle found in birds. Muscle of the highest 
quality—capable of an enormous number of very 
rapid contractions in rapid succession—consists of 
fibres in which the fine striation in the direction of 
the pull of the muscle is much obscured by irregular, 
highly granulated cross ridges. In colour it is a rich 
red. The fibres consist of small, contractible 
fibrillse, and are buried in a granular material known 
as sarcoplasm. Muscles which are capable of long- 


THE MACHINERY OF FLIGHT 


73 


continued exertion are rich in sarcoplasm, which 
would seem somehow to supply food to the muscle 
proper, to the contractile fibrillse ; at any rate this 
is a reasonable theory that explains the facts.* 
Now the muscle which in strong-flying birds is 
reddest and, in section, most ridged and granulated, 
is the Great Pectoral that lowers the wing. This 
is a muscle of great size, as I have already pointed 
out, and its output of unflagging energy is truly 
astonishing. The Elevator muscle is paler and 
smoother, and almost certainly of far inferior quality. 
Much less is demanded of it. As soon as the bird 
has got up pace, the resistance of the air lifts the 
wing, and the Elevator has little to do. In fact the 
great Depressor may claim to lift the wing, since to 
it is due the velocity which relieves the Elevator of 
its work. In the Blackcock, the Red Grouse and 
the Ptarmigan, the paleness of the Elevator forms 
a striking contrast against the rich red of the great 
muscle under which it lies. Among birds whose 
flight muscles I have examined, the Guillemot is 
the only one whose Elevator is as rich a red and 
almost as stringy as the Depressor. Is this explained 
by the fact that the bird uses his wings in swimming ? 
Has he to raise his wings in the water by muscular 
effort ? And is this due to the fact that when he 
swims he has less pace than when he flies ? In the 
Chicken, since it seldom uses its wings, all the 
Pectoral muscles are very pale and smooth. The 
wild Jungle Fowl, from which our domestic bird 
springs, uses its legs far more than its wings, and, I 
believe, domestication has not much altered the 
* See Starling’s Physiology on quality of muscle (pp. 87, 88). 


74 


THE FLIGHT OF BIRDS 


character of the different muscles. Remarkable also 
is the large size of the Chicken’s Elevator muscle. 
Its wild kinsman, haunting the jungle as it does, has 
to rise to its perch without any wind to help it, so 
that all the work of raising the wing must fall upon 
the muscle. But the effort required is not a pro¬ 
longed one, and so the muscle is pale. In the 
Sparrow-Hawk and, I believe, in other birds of prey, 
the Elevator is very small. Since they usually start 
to fly from a perch at some height above the ground, 
there is no difficulty in getting up speed enough to 
make the office of the Elevator largely a sinecure. 

The brown flesh on a Chicken’s leg, though the 
human palate pronounces it inferior, is, nevertheless, 
when regarded as muscle, of distinctly better quality. 
The Fowl is primarily a runner, not a flyer ; hence, 
presumably, the darker colour and the greater 
strength and endurance of the leg muscles. More¬ 
over, these muscles are kept on the strain throughout 
the night; by the bending of the knee and ankle 
joints the machinery of muscles and tendons through¬ 
out the leg is set to work and the bird’s toes strongly 
grip the perch. His weight keeps the legs bent, and 
the bending of the legs keeps muscles and tendons 
to their work. Were they to relax for a moment the 
bird would probably lose his life; he would fall to the 
ground and some hungry carnivore, that happened 
to be prowling about, would seize him. But though 
in the Fowl the leg muscles are darker in colour than 
those of the breast and wings, they are not in section 
ridged and granulated like the great Pectoral of 
birds that are strong flyers. In the Moorhen, I have 
noted that the leg muscles are rather more ridged 


THE MACHINERY OF FLIGHT 


75 


than is the Elevator, but not so much as the De¬ 
pressor. In most birds that I have examined, I have 
found that the leg muscles show less ridging and 
granulation even than the Elevator. But the 
Moorhen makes great use of his legs. He is both a 
swimmer and a runner. When not swimming he is 
generally walking on the grass beside his pond or 
stream, busied with the search for worms. 

Before I leave the subject of quality of muscle I 
must point out that the inferiority of the pale to the 
red has been noticed also in thoroughbred horses. 
Mr. J. B. Robertson, in his very interesting paper 
on the Principles of Heredity applied to the Racehorse 
(p. 22), writes: “The pale fibres greatly predominate 
in the tissues of a pure sprinter, and the red fibres 
in those of a stayer.” 

The big muscles, the work of which I have 
described, are all massed upon the sternum. Even 
the muscles which bend or straighten the wing at 
the elbow spring not from the humerus or upper- 
arm bone, but from the top of the coracoid and the 
anterior end of the shoulder-blade respectively. I 
once cut off the wing of a domestic Pigeon as close 
as possible to the body and found that it scaled just 
under £ oz. The bird weighed 13£ oz. Thus the 
two wings together accounted for just under one- 
eighth of the whole. It is wonderful that such 
strength can be combined with such lightness. And 
not only is the wing, as a whole, light; what weight 
it has belongs almost entirely to the bones and muscles 
of the near part. This Pigeon’s wing balanced, 
when rested on a wire 2| inches from its base 
and 10 inches from its tip. The great primary 


76 


THE FLIGHT OF BIRDS 


feathers (i.e. those which arise from the hand) point 
mainly outward and only slightly backward. Hence 
the length of a bird’s wing is due largely to feathers, 
and they are proverbially light things. Their 
strength is no less remarkable than their lightness. 

The Scaffolding of the Wing. Pneumatic Bones. 

The scaffolding of the wing is itself very light. 
The thickest of the bones, the humerus, is hollow in 
big birds that are strong on the wing. In some the 
bones are hollow right on to the finger-tips ; there 
is an opening in each bone at the near end (see fig. 
22); a thin pulmonary membrane enters there, and 
thus they are filled with air that has passed through 
the lungs. The Gannet is a good example of this 
complete aeration. Many small birds, however, 
though first-rate flyers, have all their wing-bones 
solid ; the Swallow, for example. The Swift has 
only the humerus pneumatic. The differences be¬ 
tween nearly related birds are remarkable. The 
Gannet’s remarkable pneumaticity I have already 
mentioned ; his near relative, the Cormorant, has 
only the humerus pneumatic. Such examples seem 
to make it clear that aeration is an adaptation to 
the life and habits of the particular species, not an 
unvarying character firmly established in certain 
orders of birds.* 

The fact that large birds have more aeration than 
small demands explanation. But the explanation 

* Besides the wing-bones, the Gannet has the following bones 
pneumatic : The vertebrae, the greater part of the sternum 
(not the keel), the ribs, the coracoid, the ischium and the femur. 
There are also ample air-cushions beneath the skin that covers 
the breast. 


77 


THE MACHINERY OF FLIGHT 

is not far to seek. The small bird would gain but 
little in lightness by the aeration of his bones, since 
each bone consists almost entirely of its exterior 
shell. The big bird, with his stout, bulky bones, 
will gain far more. Here is a case in which we may 
appropriately quote some geometrical facts. We have 
seen above that if we take two cubes, the side of one 
of which is twice that of the other, then a face of the 
larger one is four times the area of a face of the 
smaller one, and the cubic content of the larger is 
eight times that of the smaller. Thus an increase 
in the length and girth of a bone means a far greater 
increase of the space within the outer shell. Obvi¬ 
ously, then, a big bird stands to gain more in lightness 
by the hollowing of the bones. How he manages 
to do with so very little marrow —the bones having 
only a very thin lining—is another question to which 
I hope to return (see Chap. xi). 

How much the bigness of the bone has to do with 
pneumaticity is made clearer by the facts which 
follow. If we make measurements of the humerus 
of a Skua, or other Gull—the Gulls have very little 
aeration—and of an Eagle as the representative of 
the birds which have a great deal, we find that the 
girth of the Eagle’s bone is disproportionate to the 
bird’s superiority in length of wing. And the 
explanation, no doubt, is this ; the Eagle requires 
much greater strength in his wing-bones than does 
the Gull. Even a small increase in length of wing 
means a considerable increase in the pace at which 
the extremity will move. And, as we have seen, the 
resistance of the air increases as the square of the 
velocity. It is easy to see, then, that the Eagle and 


78 THE FLIGHT OF BIRDS 

other big, long-winged birds have wing-bones of 
larger girth (of a girth, that is, disproportionate to 
their superiority in length of wing) in order that 
they may be able to bear the far greater strain put 
upon them. A proportionate increase in weight 
would, no doubt, have caused difficulty, and this 
has been obviated by the aeration of the bones. 



Humerus of (1) Skua ; (2) Rhinoceros Hombill; (3) Sea Eagle 
—actual length 6f inches. Drawn to scale. F : Foramen, where 
the bronchial membrane enters, on the upper surface. 

To bring out this point more clearly I have taken 
the humerus of the Skua as the standard, and have 
calculated what would have been the length of the 
same bone in the Sea Eagle if it had been built on 
the same lines. 

After all this I have to admit that there remains 
a very puzzling case on which I cannot throw any 
light. The Hornbills are slow, heavy flyers, they 
are not very big, and they are the most pneumatic 




THE MACHINERY OF FLIGHT 79 




Humerus. 

Aggregate length 
of wing-bones. 


Girth of 
humerus. 

Actual 

length. 

Length 
propor¬ 
tionate to 
girth. 

Actual 

length. 

Length 
propor¬ 
tionate to 
girth of 
humerus. 


Ins. 

Ins. 

Ins. 

Ins. 

Ins. 

Skua 

ft 

4f 

Oojcs 

13f 

13| 

Sea Eagle 


6f 

7 7 

l lO 

20 t V 

22^ 


of all birds. The huge hollow beak is characteristic 
of the whole skeleton (see fig. 22). 


Stiffness of Wing. Expanse of Bone. 

It is highly important that the bird’s wing when 
extended should be rigid—its bony framework, that 
is. It has elasticity where it is wanted, the elas¬ 
ticity of the great feathers, which tends, as I have 
shown, to make the bird automatically stable during 
flight. The stiffness of the scaffolding of the bird’s 
extended wing is most remarkable when we compare 
it with the human arm. If we extend one of our 
arms horizontally to its full length with the palm 
of the hand downward, we can, while still keeping 
the upper arm lifted, bend the elbow-joint and 
point the fore-arm vertically upward. As the 
bird’s wing descends with great velocity through 
the air, it does not give at the elbow, but remains 
rigid. But the stiffness of the bird’s wrist is still 
more striking ; when the wing is extended hardly 
any up-and-down movement at that point can 










80 


THE FLIGHT OF BIRDS 


take place. The rigidity there is more easily 
brought about owing to the fact that of the two 
small rows of wrist-bones the further one has been 
fused with the hand-bones. When the fore-arm 
is brought into line with the upper arm, the hand 
also falls almost into line and is held tight in that 
position by ligaments. As soon as the wing is 
flexed at the elbow, it bends also at the wrist— 
bends backward, that is, towards where digit 5 



Skeleton of wing of Adjutant (from the elbow). D 1, 2, 3 : 
Digits 1, 2, 3. MC 1, 2, 3 : Metacarpals 1, 2, 3. R : Radius. 
RC : Radial carpal. U1 : Ulna. UC : Ulnar carpal. The 
marks left by the little pockets in which the secondary feathers 
are planted can be seen on the ulna. 

would be if it existed—and then at the latter joint 
much freer upward movement also is possible. And 
so the wing is rigid when rigidity is wanted, and 
pliant when pliancy is in demand (see fig. 23). 







THE MACHINERY OF FLIGHT 


81 


It is essential, too, that there should be a large 
expanse of bone, a firm foothold for the great flight- 
feathers. The lizard, or lizard-like reptile, from which 
birds are undoubtedly descended, had a very wobbly 
hand. Even the Archaeopteryx, the most ancient 
of birds, though it was beyond doubt a true bird and 
not a bird-like lizard, had three long fingers which 
show no sign of becoming fused together. With 
birds as we know them, things are very different. 
The three surviving metacarpals, or hand-bones, 
enlarged by the fusing with them of the further 
row of carpals or wrist-bones, are themselves fused 
together, the two of the three that are the most 
important fused both at the near and further ends. 
Of the three surviving digits two are fused ; the 
little thumb, that supports the bastard-wing, remains 
independent and insignificant; and the result of 
all the fusing is that a broad, firm platform is 
provided for the feathers. Thus we see that the 
scaffolding of the wing when extended is remarkably 
rigid in spite of its lightness ; its wide expanse of 
feathers is remarkably elastic, and is planted on a 
broad, firm base. 

The Spreading of the Wing. 

The machinery by which the wing is spread is well 
worth study. The rapidity with which it does its 
work is wonderful, and the accomplished result is 
most beautiful. When the triceps muscle straightens 
the wing at the elbow-joint, it is straightened also 
at the wrist, not absolutely but nearly ; there are 
special muscles for giving the finishing touch. Of 
the two bones in the fore-arm the front one (the 


82 


THE FLIGHT OF BIRDS 


radius) is pulled towards the body when the elbow 
is straightened and this straightens the wrist. A 
further result inevitably accompanies the first 
straightening movement. There is an elastic liga¬ 
ment that stretches from the armpit to the extremity 
of the digits. Through this pass all the secondary 
flight-feathers and, in every bird in which I have 
traced its course, all the primaries (those that spring 
from the hand). Part of this ligament is shown in 



Wing viewed from below (after Alix, with a slight alteration, 
see text). AM : Muscle and tendon supporting anterior mem¬ 
brane, D1 : Digit 1. El : Elastic ligament. F : Flexor carpi 
ulnaris, dividing into two. T : Tendons connecting muscle with 
quills. 

figure 24. But it is easy to see the whole of it in 
any bird’s wing when the small covering feathers 
have been removed. The straightening of the wing 
stretches the ligament and the great flight-feathers 
spread like a fan.* (See PI. xi.) 

* There are differences in different families. In some the liga¬ 
ment, instead of being pierced by the quills, fastens only to their 
under-side. 





83 


THE MACHINERY OF FLIGHT 

There is another ligament which fastens to the 
under-side of the quills of the secondaries nearer to 
their base. Moreover, the secondaries grow back¬ 
ward, and plant themselves each in a little pocket 
of very tough fibrous tissue. These pockets are 
firmly rooted in the hinder of the two bones of the 
fore-arm (the ulna) and leave, each of them, their 
mark on it (see fig. 23). Planted thus, the second¬ 
aries, though very firmly gripped, have freedom to 
rotate. The primaries, pointing outward as they do, 
run along above the hand and finger bones and are 
held with a still stronger grip. 

It is marvellous machinery which spreads the 
wings so rapidly, and which, having all its heaviest 
parts massed upon the body or very near to it, 
leaves them splendidly light, especially near their 
extremities. But we have not yet come to the end 
of the contrivances. During the down-stroke the 
secondaries press each against the one above it and 
so prevent the passage of air, whereas during the 
upstroke they let it pass. They are held fast by a 
number of stays ; there are the little pockets just 
described, the ligaments and a sheet of fibrous 
tissue extending from their bases to the great liga¬ 
ment. There are also little tendons that connect 
the secondaries with a muscle which, arising from 
the further end of the humerus, attaches its other 
extremity to one of the wrist-bones (carpals) and to 
one of the metacarpals, or hand-bones, beyond (see 
fig. 24). Working unopposed it bends the wrist. 
When the wing is straightened it helps by its opposi¬ 
tion to hold the wrist-joint tight, thus strengthening 
a point where the strain of flight is much felt. When 


84 


THE FLIGHT OF BIRDS 


the wing is bent the muscle lies in a slightly curved 
form—we notice it when we are getting the meat 
from between the two long bones of a chicken’s 
wing. It is straightened out when the wing 
straightens, and this, combined with the outward 
slope assumed by the feathers as they spread, 
stretches the little tendons that arise from it and 
are fastened to the feathers. The tendons slope 
outward, away from the shoulder, and attach to 
the under-side of the quills, or rather to the 
fibrous tissue with which the quills are surrounded. 
M. Alix (in his Appareil locomoteur des Oiseaux) makes 
some of them at any rate attach to the further edge 
of the lower face of the quills, and gives them the 
credit for rotating the feathers during the down- 
stroke, so that they press against one another and 
make the wing impervious to air. But when I have 
scraped these little tendons away in the wing of a 
freshly-killed Pigeon I have found that, when I 
extended the wing, the feathers still took their 
proper position. The extending of the wing causes a 
stretching of all the important stays—the ligaments, 
the sheet of fibrous tissue, and these tendons—and 
the ligaments may claim the largest share of the 
credit for the spreading and marshalling of the 
feathers. During the up-stroke the secondaries 
are firmly tethered, but are no longer marshalled 
for action. There are interspaces as there are 
between the leaves of a folded fan that has seen 
much service. The feathers being now no longer 
pressed tight together, the air can pass between 
them. When, however, the bird is flying fast and 
the wings are lifted, not so much by muscles 







PLATE'[XI. 



A : Wing of Lark, showing the elastic ligament that holds the 
primary and secondary feathers. Another ligament nearer to 
the base is faintly shown. B : Primary wing feathers of Black 
Vulture (on left) and Pink-footed Goose, upper surface ; of 
Heron, under surface ; and outer tail-feather of Heron. In all 
the inner web is the broader. (See Chap. VII.) 


[To face p. 85. 




THE MACHINERY OF FLIGHT 


85 


as by the resistance of the air, the feathers during 
the up-stroke are pressed against one another and 
the passage of the air is prevented. But this causes 
no trouble. The wing is very rapidly swept back¬ 
ward and upward ; it turns its front edge in the 
direction of flight, and when it is oriented thus it is 
easily straightened and moved forward into position 
for beginning the next down-stroke. 

During the down-stroke the air helps the living 
machinery in its work, making it still more effective. 
The outer webs of the feathers (see PI. xi) are very 
narrow compared with the inner ones, and the result 
is that the air acts much more strongly upon the 
latter during the down-stroke and also, when there 
is much momentum, during the up-stroke, with the 
result that each feather is rotated and has the inner 
side of its vane pressed very closely against the one 
that lies next to it and above it on the side nearer 
to the body. 

Though on the surfaces of the wing nothing is 
visible but feathers, yet for no small amount of the 
expanse two membranes can claim the credit. What 
is called the anterior membrane stretches from the 
head of the clavicle (or merrythought) to the hand 
(see fig. 24). In the Gannet it is of great breadth 
and is so hung that it not only increases the area of 
the wing, but sloping steeply downward, as it does, 
to form the front margin, it deepens the wing’s 
concavity and makes it, near to the body, what a 
parachute should be. In most birds the membrane 
is slung with only a gentle slope from its front edge 
backward. In all it is at once stretched when the 
wing is spread. The other membrane lies farther 


86 THE FLIGHT OF BIRDS 

back, in the armpit, and fastens the wing to the 
bird’s side. 

It is worth while noticing that though muscles 
springing from the body in the main control the 
movements of the whole wing, yet there is a good 
deal of local independence. There are niceties of 
adjustment which depend on local muscles. Though 
the triceps extends the upper arm, the fore-arm 
and the hand, yet the two united fingers, and with 
them the great feathers they carry, are not under 
its sway but depend upon special muscles to spread 
them to the full and carry out minor movements. 
The little bastard-wing has also its own muscles— 
more muscles than one would expect to be at the 
service of so insignificant a piece of machinery. 

Structure of a Flight-Feather. 

For the bird flight without feathers is obviously 
an impossibility. If the scales of the bird’s reptilian 
ancestors had remained mere scales, the ancestors 
would have remained reptiles still, condemned to 
crawl the earth. The scale has been glorified till 
it is hardly recognisable. First comes the quill with 
the dried remains of the pulp—the pulp that was 
there when it was alive and growing—still visible 
within it. Above is the rachis, or shaft, grooved 
down its front face (see Pis. xi and xn). From 
the shaft spring the barbs sloping towards the tip of 
the feather, from the barbs branch out the barbules 
or radii. From those of the barbules that are on 
the far side of the barb (the side farther from the 
base of the feather) spring the barbicels (diminutive 
of a diminutive !) that fasten barbule to barbule and 


PLATE, XII. 



(From Life and Evolution.) 

The barbs, barbules and barbicels of a flight-feather. A and 
C x 90 diameters ; B x 250. A : A barb. C : A pair of barbs 
showing interlocking. B : A pair of barbules, showing the barbi¬ 
cels on the one nearer to the tip of the feather. 


To face p 86.] 













































































THE MACHINERY OF FLIGHT 


87 


thus give the feather its elasticity, besides making it 
impervious to air. Compare for a moment a great 
flight-feather with one of those whose business is 
merely to clothe the bird and retain its animal heat 
—a poor, weak, fluffy thing. The barbicels have 
another name, hamuli or hooklets, from their 
shape. On the side of the barb which is nearer to 
the base of the feather, the barbules have no bar¬ 
bicels, but their endings resemble hairs, and these 
hair-like endings are neatly folded together so as to 
form a kind of hem. The opposite set of barbules 
lie over these smooth-ending ones, and among them 
they insert their hooklets. The hooklets slide along 
the barbules at moments of strain and stress, and to 
this, very largely, must be due the elasticity of the 
feather. The number of barbules is enormous; 
according to Dr. Gadow, over a million in one large 
flight-feather. 

Here is another very interesting point: on the 
margin of the barbules opposite to that which is 
armed with barbicels are rough knobs. These are, 
I believe, the vestiges of barbicels which, being 
useless, are in process of disappearing. The feather 
is in fact thrice pinnate. The main shaft branches 
into barbs, the barbs into barbules, the barbules 
formerly branched, I believe, into barbicels on either 
side. But the barbules on the side nearer to the base 
of the feather have altogether lost their barbicels 
and have now only hair-like endings, while the other 
set have only one member of each pair properly 
developed, the other member being represented only 
by a mere vestige, so that there are only rough knobs 
to match the barbicels. 


88 


THE FLIGHT OF BIRDS 


From whatever point of view we look at it, a flight- 
feather is a wonderful thing. It provides a large 
expanse to support the bird’s weight, it is elastic, 
it is light and at the same time strong. Moreover, 
it is renewed every year, and in such a way that the 
bird does not even for a day lose his power of flight. 

Moulting. 

A living machine differs from a man-made machine 
in many ways, and notably in this, that it has as an 
indispensable characteristic, the power of self-repair. 
When a bird’s feathers are broken or worn out, they 
must somehow be replaced. Now nearly all birds 
that fly shed their flight-feathers gradually and in 
pairs, so that, though during the moult they are not 
at their best, yet they can always rise on the wing 
{see PI. viii, 4). The Goose has somehow earned 
a widespread reputation for stupidity, but the most 
stupid thing that he does is not, I believe, generally 
known. He moults so rapidly that for a time he 
is reduced to helplessness. In the island of Kolguev, 
the Samoyeds drive thousands of moulting geese, 
who can only swim or run, into great nets and thus 
provide themselves with a store of food for the Arctic 
winter. 

But, after all, this rapid method of moulting would 
involve but little danger for the Goose if man had 
not arrived on the scene, since his way of life became 
stereotyped. Many birds, not geese alone, have 
failed to find any means of escape from their enemy 
with his ever-changing method of attack. There 
are other birds which shed all the primary-feathers 
simultaneously without any disastrous consequences. 


THE MACHINERY OF FLIGHT 


e.g. the Ducks. Mr. J. L. Bonhote has made a 
special study of the subject, and, quoting from him, 
I am able to give some very interesting and instruc¬ 
tive facts. The simultaneous moulting of the 
primaries occurs among the Divers, the Guillemots, 
the Razorbills, the Puffins, the Auks, and besides 
these among the Rails, e.g. the Moor-hens, the Land¬ 
rails, and the Crakes. Nearly all of them are water 
birds, and in the water they are fairly safe from 
their ordinary enemies. Some of them can dive, 
others can hide among reeds or under bushes; the 
Geese by force of numbers can, probably, beat off 
a bird of prey. A sea bird, if he is to moult in this 
fashion, must be a true diver in habit and in plumage, 
if not in name, and thus we find that the Cormorant 
(who is always wanting to dry his wings on a rock) 
and the Gannet moult gradually. When we turn to 
land birds the case of the Corncrake does not surprise 
us, for though he is a migrant, yet in his day-to-day 
life he trusts more to his legs than to his wings. 
The case of the Coot is perplexing ; unlike the Moor¬ 
hen, he is capable of flight throughout his moult; 
but, as Mr. Bonhote points out, he is the only member 
of the Rail family that frequents estuaries and open 
sheets of water, and he is, in addition, an indifferent 
diver. * 

Legs. 

I have already pointed out how some birds use 
their legs in balancing and, possibly, for steering. 
Indirectly the great leg-power possessed by most 
birds is a most important aid to flight. In order to 

* See “ Eclipse Plumage and Flightlessness ” by J. L. Bonhote, 
in the Field, March 24, 1906. 


90 


THE FLIGHT OF BIRDS 


rise with ease from the ground, they must be able 
first to jump into the air. Most birds have an active 
springing gait, and are good jumpers, their legs being 
built much in the same way as those of a horse or 
an antelope ; the ankle-joint is raised high above 
the ground, and they walk or run upon their toes. 
The springiness is combined with remarkable light¬ 
ness. The bird’s foot is made almost entirely of 
skin, bone, and tendon ; not like the human foot, 
fleshy and full of nerves. It is worked by long 
tendons attached to muscles that spring from the 
top of the leg-bone (the tibio-tarsus) or even from the 
base of the thigh-bone, and, running in grooves under 
the ankle-joint, give to the foot its springiness and 
to the toes their wonderful grip. The fusion of the 
two rows of ankle-bones with their bigger neighbours 
makes the ankle-joint an excellent pulley. Alto¬ 
gether the bird’s wings have in the legs very able 
assistants. Comparatively few resemble the Swift 
in having very short and feeble legs ; but, as I have 
pointed out above, there are a considerable number, 
mostly big, bulky birds, which, owing to their 
shortness of leg and small power of jumping, have 
difficulty in starting to fly from level ground. 

















































































































PLATE X1JL1. 



A: Wing of hen Pheasant, from above ; B, from below. Actual length of wing photographed, Ilf inches. 
C : Wing of Curlew, from below ; actual length, 19 in. D : Wing of cock Sparrow-Hawk, from below 7 ; 
actual length Ilf in. B, C, D are intended to show the amount of curve or camber, and the curved line of 

the back edge of the wing. (See Chap. VIII.) 



CHAPTER VIII. 


VARIETIES OF WING AND OF FLIGHT. 

CURVE-NARROW AND BROAD WINGS-STYLES OF FLIGHT- 

FLIGHT IN FLOCKS-THE WHIR OF WINGS. 

Curve. Narrow and Broad Wings. 

I shall first consider the question of curve, or, as 
the aviators call it, of camber—a question of the 
utmost importance. The wings of all birds are a 
good deal curved from front to back, so that as they 
descend they catch the air in a concavity and have 
plenty of lifting power. In some birds the front-to- 
back curve extends throughout the whole length of 
the wing, though in all there is some shallowing 
towards the tip. The Jay and the Red-legged 
Partridge (and their allies) supply good examples of 
a curve that shallows near the tip of the wing but 
does not disappear. In a Jay, of whose wings I made 
measurements, the depth of the curve—measured 
just on the near side of the starting-point of the 
bastard-wing—was 1 inch (see PI. xv and also xiii 
and xiv). The breadth of the wing at this point 
was 5| inches. Thus the curve is 1 in 5§, a curve 
such as no aviator would ever think of using. But 
it must be remembered that during the down-stroke 
it is much reduced by the pliancy of the feathers, 
which yield to the pressure of the air. The span 


92 


THE FLIGHT OF BIRDS 


from wing-tip to wing-tip was only 18 inches. In 
fact it was a very short, broad, much-curved wing. 
The Pheasant’s wing belongs to the same class ; it 
is very short (in a hen bird of which I took measure¬ 
ments, only 20 \ inches); very broad (6 inches); 
and the curve in this particular bird was no less than 
\\ inches—1J in 6 ! (see PI. xiii). 

The Hoopoe’s wing is very much of the same build 
—unfortunately I cannot give exact measurements 
—and yet, when the season of migration comes 
round, many Hoopoes manage somehow to flap 
across the Mediterranean. As a rule migrating birds 
are characterized by wings of a very different make, 
and it sometimes happens that the wing is the 
feature in which they present the most striking 
contrast to their non-migratory kin. The clamorous 
Reed-Warbler, that is resident in Egypt, shows by 
his wings that he does not migrate—they are so very 
short. The migrant Reed-Warblers have decidedly 
longer and more pointed wings. As to the Hoopoe 
and its short, broad wings, it must be remembered 
that though it manages to cross the Mediterranean 
when the conditions are favourable, yet its migration 
flights are comparatively short. Some, I believe, 
do not migrate at all. If they are delayed by clouds 
while crossing the Mediterranean, their strength is 
apt to give out, whereas Herons and Swallows, to 
take two examples, have a reserve of vitality and 
will fly round a steamboat for hours, till, perhaps, 
the weather clears again, when with unflagging 
strength they will continue their journey. 

To pass on to the Red-legged Partridge, I found 
the depth of the curve to be 1 inch, as in the Jay, but 


PLATE XIV. 



Wings, from above. A : Gannet (actual length, 31 in.) and Montagu’s Harrier. B : Of Tern and Herring- 
Gull (actual length, 22 in.). C : Of Lapwing (actual length, 14 in.). D : Of Hoopoe (actual length, 74 in.). 

{See Chap. VIII.) 

To face p.,92.] 






















VARIETIES OF WING AND OF FLIGHT 93 

the width of the wing was only 4| inches, whereas 
the Jay’s was 5f inches wide. The wing-tip to wing- 
tip measurements were 18 J inches, hardly an advance 
upon the Jay. It is a very short wing, and though 
narrower than the Jay’s, it is a broad wing if we 
compare it with those of most birds. The Moor-hen, 
also, has a rounded and not very long wing, with a 
good deal of front-to-back curve, shallowing, however, 
very much near the extremity. The measurements 
in one specimen were : curve, inch ; breadth, 
4J inches ; wing-tip to wing-tip, 21J inches. The 
breadth is very little greater than in the case of the 
Red-legged Partridge ; in length the Moor-hen has 
decidedly the advantage, and the gaps between the 
extremities of the feathers are much less ; in other 
words, the wing is more finished and more efficient. 
Here in England we look upon the Moor-hen as a 
stay-at-home bird, but it will cross the Alps in 
search of a warmer climate if need be. Frozen-out 
Moor-hens from northern or central Europe often 
accomplish this feat. 

In other birds we find the curve much more reduced 
tow r ards the extremity of the wing ; in some it dis¬ 
appears almost entirely. In the Thrush the last 
inch and a half is nearly flat. In the Starling 
(PI. xv) there are quite 3 inches with very little 
curve. To take a bird of larger build, the Curlew’s 
wing (PI. xm) shows a great reduction of curve 
as soon as it begins to taper to a point; the last 
4 inches are nearly flat. Moreover the curve has 
not the same character throughout. For a distance 
of some 9 inches from the body, a front margin, nearly 
an inch broad, is flat; behind that the downward 


94 


THE FLIGHT OF BIRDS 


curve begins. Further out the whole breadth of 
wing is curved ; for some distance from the front 
margin there is an upward slope, then a downward 
slope sets in. In the nearer region, also, there is not 
a simple curve maintained throughout, but the 
feathers in one part are more bent down than in 
another, so that the back edge presents a curiously 
undulating line. The Sparrow-Hawk’s wing has 
similar curves, but they are much less pronounced. 
‘What the exact significance of all this complication 
may be, presenting so marked a contrast to the 
uniform curves of an aeroplane, it is difficult to say. 
Probably there is some significance, since in birds 
that are strong on the wing the whole mechanism 
of flight down to minute details is so efficient. 

Sometimes we may see the wing-curves in their 
full beauty in a live bird, but it is only for a moment, 
and one wishes to look at them quietly and study 
them. The thing is, if one has a freshly-killed 
specimen, to cut off the wings close to the body and 
pin them out back downwards at their full stretch. 
Treated thus the wing retains its curves and a great 
deal of its beauty. If it is pinned face downwards 
and flattened, it conveys by no means so good an 
idea of what it was during the life of the bird. I 
have tried by photographs to show the outlines of 
various types of wing, the depth of their concavities, 
and the undulating curves seen in, for instance, the 
Curlew’s wings. Some points come out distinctly, 
others the camera fails to see or does not see clearly. 

To show the gradual tapering and flattening of 
the farther half, perhaps no wings are better than 
those of the Gannet and the Tern (PI. xiv), and 


PLATE XV 



A and B : Freshly-killed birds suspended by the wing. A : Starling (actual length of wing, in.). B : 
Jay (actual length of wing, 9| in.). C : Acrocephalns stentoreus. Clamorous Reed-Warbler (7 in. from 
wing-tip Ito wing-tip when wings are partly flexed, as in photograph). (See Chap. VIII.) 













VARIETIES OF WING AND OF FLIGHT 95 


they are things of wonderful beauty. We are 
probably right in drawing from this flattening the 
inference that the swifter movement of this part of 
the wing makes a deep curve undesirable. The near 
part moves more slowly and is more parachute-like 
in character. The farther part, besides the momen¬ 
tum of the bird as a whole, has the great rapidity 
of stroke which sends it with a sudden dash both 
forward and downward. We can hardly doubt that 
the short, rounded wing is the primitive one. The 
wing of Archaeopteryx, most ancient of known 
birds, did not taper to a point. And the clumsy 
flyers—those with rounded wings have a compara¬ 
tively feeble flight—must have preceded the skilled 
flyers. The Hoatzin, with his very short, rounded 
wing, cuts a very poor figure in the air. The Hoopoe 
and the Jay are not strong flyers. It is not only 
that their wings are short and very broad: there 
are great interspaces, towards the hinder margin, 
between the feathers. This, as I have shown above, 
may aid automatically the maintenance of equili¬ 
brium, but the gaps are wider and deeper than are 
necessary for this purpose. In fact the Hoatzin’s, 
the Jay’s and the Hoopoe’s wings suggest the 
work of a “ ’prentice hand.” 

The narrow wing is certainly not primitive. I 
have shown above (see Chap, n) that most of the 
work is done by the front part of the wings, and this 
becomes increasingly true when the wing moves 
very much forward, cutting the air at a small angle 
with the horizon. This is the way in which the long 
wings of the best flyers cut the air. And it is in 
these best of flyers (I am not speaking of the Soarers) 


96 


THE FLIGHT OF BIRDS 


that we find the breadth of the wing much reduced ; 
they taper towards the extremity where the rapidity 
reaches its maximum and the forward movement is 
greatest. 

Styles of Flight. 

There are two contrasted styles of flight that even 
the casual observer easily distinguishes. There is 
that of the small bird that lifts himself with two or 
three rapid strokes, then takes a rest and glides 
onwards, his wings as a rule not quite at their 
full span—glides so far that though his pace is con¬ 
siderable he loses not a little altitude. But a few 
strong strokes soon make this good, and he enjoys 
another slightly-downward glide. The Swift and 
the Swallow are first-rate exponents of this style, 
but it is common to most small birds. The Water- 
Ouzel is a striking exception ; his wings move so 
fast that you see only a blur, and he allows himself 
no easy intervals of gliding. Probably this is 
because, for his size and weight, his wings are 
decidedly small, so that he has to ply them unceas¬ 
ingly. When a larger bird flies in the dipping style 
■—a few strokes and a glide, a few strokes and another 
glide—it is particularly striking. Everyone knows 
the dipping flight of the Woodpecker. When he 
glides, he flexes his wings more than most of the 
birds that intersperse little glides amid their flight, 
and so he dips more than others. Sometimes a bird 
whose wings are large for his size and who gains 
much altitude with a stroke or two, then half folds 
his wings, has a very butterfly-like appearance. The 
Wallcreeper, when he plays about a vertical face of 
rock in the Alps, spreading and then half flexing his 


VARIETIES OF WING AND OF FLIGHT 97 

wings with their rich red coverts, looks like a butter¬ 
fly, and a butterfly of marvellous beauty. 

The reason that birds of larger build do not fly in 
this style is, probably, as I have pointed out above, 
that their rate of stroke being slower they lose 
altitude while raising their wings, unless they have 
considerable momentum. Their best policy, there¬ 
fore, is to attain and maintain great pace, rather 
than to gain altitude and then indulge in a slightly 
downward glide. Moreover, if a big, bulky bird 
were to set his body at a suitable incline for rising 
almost vertically, as a Pigeon sometimes rises, his 
wings, turning stiffly as they do at the shoulder- 
joint, would not be able to beat in the right direction 
and would drive him backward rather than lift him. 
Some rather big birds do a good deal of gliding, but 
it is a very different performance from that of the 
small bird. They first gain altitude, not, however, 
by only two or three strokes, as the small bird does, 
but by a number. In fact they get up momentum 
and then are lifted, as an aeroplane set at a slight 
incline to the horizon is lifted when it is driven 
rapidly forward. They will then glide onward with 
wings outstretched to the full, so as to lose as little 
altitude as possible. A wide spread of wing is the 
thing needed, for, since it is the front part of the wing 
on which the air mainly acts, the greater the front 
presented the greater the supporting power. The 
big bird’s gliding is very unlike that of the small bird 
that knows how easily by three or four strokes he 
can recover elevation, and therefore quickens his 
glide by the sacrifice of some of that which he 
has already gained. Let us take as examples of 


98 


THE FLIGHT OF BIRDS 


comparatively big birds the Grouse and the Partridge. 
The great concavities beneath their wings, no doubt, 
aid them much. But the intervals of gliding, 
depending as they do on the preliminary attainment 
of great momentum, necessarily come at rarer 
intervals than they do in the case of the small bird. 
I am not speaking here of the long glides achieved 
without loss of altitude by some large, or fairly large, 
birds when the wind has an upward trend. That is 
a feat of the same nature as soaring, and I reserve 
it for another chapter. 

In another way the big and the small bird present 
a striking contrast. The former is often the slave 
of his own momentum, and only with difficulty and 
effort deviates from his line of advance ; the Goose 
and the Duck are good examples of this. The 
Swallow, on the other hand, turns with the utmost 
agility and ease. Some bigger birds have a wonder¬ 
ful nimbleness, though, in this respect, I think, they 
are no match for the Swallow. The Sparrow-Hawk, 
as he pursues some coveted small bird, turns and 
twists among the trees and bushes with great skill, 
his long,broad tail helping him very much in steering. 
Birds of prey must, of course, have great nimbleness, 
great power of suddenly checking themselves or sud¬ 
denly changing their course; otherwise they would 
be unsuccessful as hunters, and would dash themselves 
to pieces when swooping upon some intended victim 
that was flying not far above the ground or a tree. 
The Lapwing turns to account his power of putting 
on the brake and making sudden turns by dashing 
at his enemy, alarming him, then suddenly pulling 
up and retreating only to make another dash. The 


VARIETIES OF WING AND OF FLIGHT 99 

great breadth of his wings at their outer extremities 
may, as I have said, account for his well-known 
tactics and the skill with which he carries them out 
(see PI. xiv). 

The pace of the stroke varies very much in different 
species, a subject on which I have already said 
something.* A Stork goes along in very leisurely 
style, taking no more strokes per minute than a 
Heron, i.e. some 130, or even less. But one must 
not imagine that little force is being used. The 
Stork’s wings are very long, and the upward bending 
of the primary feathers shows the great rapidity 
with which they are being driven through the air. 
It is the birds with big, long wings that have the 
slowest stroke. The great length of wing makes 
each stroke very effective; and the slow beats that we 
see so easily are not at all slow towards the wings’ 
farther end. Heavy birds that have short wings 
have to take very rapid and Very long strokes; the 
wing is lifted fairly high in horizontal flight, very 
high when they are rising, and descends till the tip 
has described a large segment of a circle. The Duck 
is a familiar instance of this style of flight. His long, 
strong strokes send him hurtling through the air 
with ponderous momentum. The same style of 
flight is carried to its extreme by the Guillemots, 
Razorbills, Puffins, and other diving birds which use 
their wings in swimming. It would be no good trying 
to fly under water with big, long wings ; after the 
completion of a stroke, to get the wing back into 
position for the next would be a difficult operation. 
These birds, then, have the largest wings with which 
* See p. 50. 


H 2 


100 


THE FLIGHT OF BIRDS 


it is possible for them to swim, and the smallest with 
which it is possible for them to fly. The wing-beats 
during flight are marvellously rapid ; there is visible 
a rapidly-moving body and a blur on either side 
where the wings are working with terrific speed. 
The Water-Ouzel searches for his food among the 
weeds in the bed of fast-running streams, and he, 
too, has short wings that he plies with very great 
rapidity. 

The rolling flight of Partridges and Grouse is very 
striking. When you put them up, they never, till 
they have got some distance off, keep on an even 
keel. Probably this rolling flight makes it less easy 
for big birds of prey to swoop down upon them with 
true aim, and it may help them to glance at a 
pursuing enemy. In fact, the Partridge and the 
Grouse are still, in their habits, adapting themselves 
to an environment of raptorial birds rather than to 
an environment of sportsmen and gamekeepers. 
The zigzagging flight of the Snipe—a very spirited 
and characteristic performance—is a different thing; 
the line of flight is a zigzag, but at the same time 
there is the rolling that we see in the Grouse’s flight. 
It is well calculated to confuse the aim of an assailant, 
whether he be a bird of prey or a novice with a gun. 

The way in which the long-necked and the long- 
legged birds carry their necks and their legs is inter¬ 
esting. The Duck and the Goose stretch their necks 
forward to their full length. This may be because 
the breastbone that carries the heavy flight-muscles 
is very long, so that the head and neck are used to 
balance the weight farther back. The Heron flies 
habitually with his neck bent—a remarkable fact 


VARIETIES OF WING AND OFJFLIGHT 101 

if we bear in mind that he carries his long legs 
stretched out behind. But his breastbone is short 
and deep, so that the great weight of muscle lies 
forward. Occasionally he may be seen to extend 
his neck full length, apparently for balancing pur¬ 
poses. The Stork is long-legged and the Flamingo 
still more long-legged, and both of them carry their 
necks and legs straight out. This is the normal 
attitude also of birds of prey. The Stilt, too, flies 
with his marvellous legs streaming out behind him ; 
his style of flight is well shown in a drawing in Mr. 
Abel Chapman’s Wild Spain. 

Flight in Flocks. 

When birds fly in a flock, great or small, they often 
adopt a particular formation, very commonly that 
of a V, and this is sometimes spoken of as if it had 
some special merit. But really the only important 
thing for each bird is to keep clear of the wash of 
his predecessor and the broken columns of air that 
he leaves behind him. The force of the wash is the 
measure of the vigour a bird puts into his wing- 
strokes ; it will not do, therefore, to travel close in 
the wake of another bird. The bicyclist, on the 
other hand, breaks the resistance of the air, and the 
man who rides close behind another has an advantage 
since he finds the air more yielding. As the man in 
front is not riding on the air there is no back-current 
behind him (see p. 4). 

The Whir of Wings. 

Everyone must have noticed the different notes 
given out by the wings of different kinds of birds as 


102 


THE FLIGHT OF BIRDS 


they fly. Very striking is the contrast between the 
shrill whistling of the wings of the slow-flapping 
Swan and the whir or swishing sound that accom¬ 
panies the flight of a flock of Starlings. It has been 
thought that the particular note depended on the 
pace at which the wing moved through the air, but 
in reality it is due to the vibration of the feathers. 

Wing music is often very beautiful; it is a grand 
thing to get near to a large flock of Golden Plover 
and hear and see them go scudding by. There is 
also pleasure in watching an Owl when he comes 
near, and in realizing the complete silence of his 
flight. At the edge of the outer web of his first 
primary wing-feather, the hooklets of the barbules 
are missing, and the barbules themselves are mere 
vestiges. Consequently the edge of the feather has 
the softness of down, and this, no doubt, has much 
to do with the ghost-like silence of the Owl’s flight. 
The Nightjar also is a very silent flyer, though in 
his wing-feathers the hooklets are nowhere missing. 
Still his plumage is remarkable for its softness, and 
this probably accounts for the absence of whir as 
he plies his wings. 


CHAPTER IX. 


PACE AND LAST. 

EXPERIMENTS AND OBSERVATIONS—WIND—VELOCITY OF 
MIGRATORY FLIGHTS—ENDURANCE. 

Many and various are the ways that have been 
tried of measuring the velocity of the flight of birds, 
and, unfortunately, the various ways lead us to 
divergent conclusions. Some years ago some experi¬ 
ments of indisputable accuracy were made in a 
range constructed for experimental shooting. Two 
“ screens ” formed of very fine threads were put up 
at a distance of forty yards from one another. These 
screens were connected with electrical apparatus, 
by means of which the time occupied by the bird 
in traversing the forty yards was registered. The 
highest speed attained by any of the twelve Pigeons 
experimented on was 33*8 miles per hour, the lowest 
26* 1. Similar experiments have been made in the 
open. The velocity of four Pigeons was measured 
on a calm day by persons stationed at a certain 
distance from one another, who marked carefully 
the moment at which the birds came opposite to 
them and registered it with a stop-watch. The 
fastest travelled at the rate of 27*9 miles per hour. 
Pheasants were experimented on in the same way 
in the range and in the open ; in the former case the 
velocity was 33*8, in the latter 36*1 miles per hour. 


104 


THE FLIGHT OF BIRDS 


These outdoor observations may be inaccurate, but 
we can hardly cavil at the experiments in the shooting 
range.* The verdict, is, indeed, a startling one. If 
this is all they can do, birds have a reputation for 
far greater pace than is warranted by the facts. They 
are thought to be far swifter than the swiftest horse, 
but Ladas’s time over the Derby course, 1J miles, 
gives him a velocity of 32while Spearmint in 1906 
won the Derby in 2 minutes 38*8 seconds ; i.e. he 
maintained a speed of just over 34 miles per hour. 

However, a thoroughly competent observer, Com¬ 
mander H. Lynes, has made similar experiments 
which give decidedly different results. In his 
observations on the Migration of Birds in the Mediter¬ 
ranean (British Birds , Vol. hi), he writes : “ The 
only passage speeds I was able to deal with were 
those of some of the species which arrived flying 
low. The best observations were made on the 
Quails by timing them from the moment they crossed 
the fore-and-aft line of the ship to the moment that, 
with a pair of glasses, they could be seen to fly into 
a quail-net exactly 500 yards distant. The result 
gave a speed of just 50 knots per hour. Corncrakes, 
Water-Rails and Spotted Crakes arriving appeared 
to be going just about the same speed, but proper 
time-observations of them were never obtained.” 

Fifty knots is just over 57 miles, a very different 
verdict from that pronounced on Pigeons by the 
experiments in the shooting-gallery and the other 
similar experiments in the open air. And yet the 

* See Charles Lancaster’s Illustrated Treatise on the Art of Shoot¬ 
ing, p. 175, and SirB. Payne-Gallwey’s Letters to Young Shooters , 
p. 152. 


PACE AND LAST 


105 


Quail, when one sees it on migration, flying past a 
steamer, does not seem to be one of the fastest flyers. 
Unfortunately, Commander Lynes does not record 
any observations made on the wind at the time the 
Quails were flying the measured distance. 

Mr. H. H. Clayton, when engaged in measuring the 
heights and velocities of clouds by means of special 
theodolites, took the opportunity of finding the pace 
and the height at which passing flocks of birds 
were flying. In one case—the birds were Ducks—he 
found the speed to be 47*8 miles per hour and the 
altitude 958 feet above the ground. Some Geese 
flew at 44*3 miles per hour at a height of 905 feet.* 
I believe Mr. Clayton gives no record of the wind. 

The records of the races of Homing Pigeons give 
a very favourable verdict. But when the “ times ” 
have been very good, there has been in almost every 
case (perhaps in every case without exception) a tail¬ 
wind to help them. I take some records from the 
Homing Fancier's Annual for 1892. For 82 miles a 
bird maintained a velocity of just over 71 miles per 
hour. The weather was “ splendid,” but the winner so 
outstripped all the others that we cannot help being 
slightly suspicious as to the correctness of the record. 
In a longer race—215J miles, from the Scilly Isles 
to its home in Wiltshire—a Pigeon kept up a speed 
of 50| miles per houi*. In a short race—only 80 
miles !—we have a velocity of 58§ miles recorded. 
In a race of moderate length—170 miles—the winner 
travelled at the rate of just over 54 miles. There 
is another record of 57 \ maintained for 104 miles. 
In all these cases, with one exception, it is recorded 
* Science, n.s., Vol. v, No. 105, p. 26. 


106 


THE FLIGHT OF BIRDS 


that there was a tail-wind to help the birds. With 
regard to the 80-mile race, the only meteorological 
note is “ weather hazy.” In the Working Homer , 
a high authority lays it down that a tail-wind is not 
absolutely essential to good “ times ” ; what is all¬ 
essential is anticyclonic weather. A cyclone is 
disastrous to them. In a light breeze during an 
anticyclone it is recorded in this treatise that a 
Homing Pigeon flew from Banff to North Hants at 
the rate of 1,900 yards per minute, or 62 miles an 
hour, an astonishing pace. The author tells of a 
celebrated bird “ Vonolel,” which in two races 
maintained a velocity of over a mile a minute. 
Unfortunately he gives no record of the weather. 

In France the experiment has been made of 
employing Swallows in place of Homing Pigeons. 
The idea is a very ancient one, for Pliny tells us that 
a certain Roman knight, who wished to let his friends 
at Volaterrae (in Tuscany) know who had won the 
chariot races, used to take with him to Rome—a 
distance of 130 miles—some Swallows, which he let 
loose after dyeing them the colour of the winner. 
Of the experiments in France I have not been able 
to obtain any accounts at first hand. One flight is 
reported to have been a very grand one, far sur¬ 
passing anything credited to a Homing Pigeon. A 
Swallow was taken from Roubaix to Paris, a distance 
of 258 kilometres, or 160 English miles, and in 90 
minutes from the time of its liberation at Paris it 
was back again. It had kept up a pace of 106 miles 
per hour! * This may seem incredible, but the 

* See an article quoted from the Globe in the Zoologist .for 1887, 
p. 397. In the Homing News for Sept. 13, 1889, is an account 
apparently of the same flight, the distance being given as 250 
kilometres. 


PACE AND LAST 


107 


figures may possibly be correct. It must be remem¬ 
bered that the Swallow is better built for rapid flight 
than the Pigeon. Of the velocity attained by the 
Swift, that in his flight is very like the Swallow, but 
certainly more than his match, many people have 
arrived at an even higher estimate. 

Wind. 

But how are we to account for the Pigeon’s com¬ 
paratively very poor pace when tested in the shooting- 
gallery ? I have shown that in most cases, when 
they make records, Homing Pigeons are aided by a 
tail-wind. Besides this it must be remembered that 
at a considerable altitude the air is more rarefied, 
and consequently offers less resistance. It is true 
that it gives less support. But a bird of strong 
flight, travelling fast, will get the support that he 
requires, so that he will gain and not be hindered 
by the rarefaction of the air. It hardly seems, 
however, that this can be the sole explanation, since 
Commander Lynes found that Quails maintained a 
velocity of 57 miles per hour when they had left the 
higher air and, on nearing a resting-place on their 
voyage, were flying low. Commander Lynes’s 
observations were made, I believe, on a number of 
Quails under varying conditions, so that it seems 
probable that we have here the bird’s own pace, not 
its pace plus that of the wind. 

The difficult complication introduced by the wind 
has been ably dealt with by Dr. Thienemann at his 
post of observation not far from Rossitten, the well- 
known ornithological observatory on the Baltic. 
He has set up two lines of stakes half a kilometre 


108 


THE FLIGHT OF BIRDS 


apart; they run at right angles to the usual stream 
of autumn migration, which flows from NN.E. to 
SS.W. He takes up his position at a point in one 
of these lines ; at the corresponding point in the 
other line his assistant is stationed. With a stop¬ 
watch he marks the moment at which a bird passes 
his post of observation: at the moment it passes the 
other post, the assistant telephones the fact to Dr. 
Thienemann, who records it on his stop-watch. The 
direction of the wind is taken and the exact angle 
it makes with the bird’s line of flight ; its pace is 
measured by means of an anemometer. With these 
data, whatever the angle may be, it is not difficult 
to calculate the distance travelled by the wind with 
the bird or in the opposite direction, while he is 
covering the measured half kilometre. After finding 
the rate at which the bird travels per second, per 
minute, per hour, Dr. Thienemann subtracts or adds 
the distance travelled by the wind, with or against, 
during the same time. Thus the disturbing wind- 
factor is eliminated and he arrives at the bird’s own 
velocity. All this is wonderfully ingenious and 
wonderfully thorough ; the results have great value 
because they are undoubtedly dependable. We can 
only regret that the species of birds observed do not 
include any of those observed by Commander Lynes. 
Moreover it seems probable that they were not flying 
at full speed. Migration, as a rule, goes on at a great 
height; these birds had descended to a low level, 
and with land beneath them were flying probably 
in leisurely style. The much greater velocity of 
Commander Lynes’s Quails is accounted for, I believe, 
if we bear two facts in mind ; they are, presumably, 


PACE AND LAST 


109 


birds of more rapid flight, and besides this, though 
they had descended from their lofty heights, they 
were, unlike the birds at Rossitten, still straining 
towards the land. But in any case Dr. Thienemann’s 
observations, in point both of method and results, 
are interesting and valuable. The pace of no less 
than twenty Grey Crows flying at different times 
under varying circumstances was measured, and the 
average velocity, after allowance had been made for 
the wind, was 50-04 kilometres, or 31*5 miles per 
hour. Two Jackdaws had an average of 39*6 miles. 
A Starling flew at the rate of 46'5. Six Finches 
averaged 33*0, two Crossbills 37'5. This is about 
what we should have expected. But these definite 
observations are much better than the shrewdest 
of guesses. 

Velocity of Migratory Flights. 

There is good reason to believe that birds while 
migrating attain far greater velocity than they do 
in their ordinary flights. Let us take an example 
for which there seems to be strong evidence, though 
it is almost too marvellous to be true. The American 
Golden Plovers breed in Arctic regions, from Alaska 
to Greenland, above the limits of forest growth, and 
when autumn comes they pass over Nova Scotia, 
strike boldly out to sea, and, generally leaving the 
Bermudas well to the west, sail on over the ocean till 
they reach the West Indies. It is difficult to believe 
that these are merely stray birds that have been 
blown out of their course and are sailing on to death. 
One witness after another declares that he has seen 
flocks of them flying southward several hundred 


110 


THE FLIGHT OF BIRDS 


miles to the east of the Bermudas, on which islands 
they alight only if the weather is unfavourable. 
Flying south from the Bermudas or somewhere east 
of them, they must cover some 1,700 miles before 
they land on one of the West India islands. Either 
then they fly at an almost incredible pace or they 
remain upon the wing an almost incredible time. If 
this wonderful flight is really achieved by the 
American Golden Plover, it is certainly the most 
wonderful athletic feat with which birds can be 
credited. But there are other flights which might 
well strain our power of belief, if the evidence were 
not so strong. There are migrant birds which pass 
the summer (the Antipodean summer) in New 
Zealand, and among them are some land birds, 
notably two species of Cuckoo. “ The long-tailed 
Cuckoo ” (Eudynamis taitensis )—I quote from Mr. 
W. L. Buller’s Manual of the Birds of New Zealand *— 
“ which is a native of the warm islands of the South 
Pacific, visits our country in the summer and breeds 
with us. It begins to arrive about the second week 
in October, but it is not numerous till the following 
month, when pairing commences.” To get to New 
Zealand from New Caledonia it must pass a very 
wide stretch of sea, and, if it can find no small island 
to use as a resting-place, it must cover 1,000 miles 
in one flight. Since it almost certainly comes, not 
from Australia, but from the islands to the north¬ 
west and north of its summer home, there is little 
Norfolk Island or the Kermadec Islands that might 
be used as convenient halting-places. There is 

* Buller’s Manual of New Zealand Birds, p. 7. See Captain 
Hutton’s Animals of New Zealand. 


PACE AND LAST 


111 


another Cuckoo (Chrysococcyx lucidus) called the 
Shining Cuckoo, which also probably follows the 
same route. Both of them pass over a very wide 
but shallow stretch of sea, from New Caledonia to 
New Zealand, a region that in the early Tertiary 
period was dry land. These birds, ultra-conservative 
like nearly all feathered creatures, are keeping up the 
practice of a great migratory flight, which, when 
first attempted, was an overland journey, but now 
passes over hundreds of miles of open sea. Captain 
Hutton and others who have studied New Zealand 
migrants are, apparently, of opinion that Norfolk 
Island and the Kermadecs are not used as pieds a 
terre, at any rate by all the birds that flock southward. 
And so they make the journey in one long flight. 
That the land birds ever rest on the sea is extremely 
unlikely, and it is, of course, impossible that they 
should ever get food from it. I here is another 
bird that certainly deserves mention here—the 
Eastern Godwit (Limosa baueri). It has been 
found nesting in Alaska and in Northern Siberia 
(74°-75° N. lat.), and in August begins, in vast 
numbers, to move southward, passing along the coast 
of Formosa. It has been observed in Norfolk Island 
—a very interesting fact. But do all the Godwits 
reach New Zealand by this route ? Even if they do, 
we must not assume that they always take a rest on 
Norfolk Island. It may be that they only pause 
there if the weather is unfavourable. It is highly 
probable that some migrant birds cross from Australia 
to New Zealand. The Australian Swallow occa¬ 
sionally makes its appearance there, and even if it 
pauses to rest on one of the Lord Howe Islands it 


112 


THE FLIGHT OF BIRDS 


has still a stretch of more than 800 miles of sea to 
fly over. Thus the New Zealand evidence, though 
it is not yet as complete as we could wish, justifies 
us in attributing to birds quite extraordinary powers 
of endurance. 

I have already pointed out that the rarefaction 
of the air at high altitudes makes it less resistant 
and less buoyant, an advantage without any draw¬ 
back. The migrant bird flying, say, some three, 
four or five thousand feet above the sea level, will 
not suffer from the attenuation of the atmosphere, 
for, since he travels with very great velocity, it will 
give him all the support he wants. But there is 
another fact that must be borne in mind which may 
make the achievements of migrant birds more 
credible. 

Homing Pigeons, as we have seen, make the best 
times as a rule when they have a tail-wind to help 
them. Migrant birds, on the other hand, very often 
have, it is said, the wind on the shoulder or blowing 
almost straight in their faces when they make their 
flights. But we must not without evidence jump to 
the conclusion that the wind at the altitude at which 
the birds are flying is blowing in the same direction 
as it does at our level. We have only to watch the 
clouds to discover that often at no very great height 
there is an upper current that is not following the 
same course as the wind below. Fifty years ago 
Mr. Glaisher made some investigations, by means of 
balloons, that threw much light on the subject. He 
found that, though the direction of the wind close 
to the earth was sometimes that of the whole mass 
of air up to 20,000 feet, yet at other times the direc- 


PACE AND LAST 


113 


tion changed no more than 500 feet up. Sometimes 
directly opposite currents were met with at different 
heights in the same ascent, and three or four streams 
of air were encountered moving in different direc¬ 
tions.* But we have other more recent observations 
at our disposal. During the last few years the upper 
atmosphere has been investigated with a thorough¬ 
ness never before attempted. Unfortunately most 
of the papers written on the subject deal more with 
the question of temperature than with wind direction. 
Often, too, they are concerned with altitudes to 
which no bird could possibly attain. But when we 
have made these deductions there still remains in 
the papers published by the Meteorological Society 
much that is of the greatest interest to the orni¬ 
thologist. I single out the records of a few striking 
observations. 

On June 23rd, 1909, kite ascents at Glossop 
brought out the fact that while the surface-wind 
was south, there was at an altitude of 2,460 feet a 
south-by-west current, f A great deal is to be learnt 
from Mr. Cave’s Pilot Balloon Observations in Bar¬ 
bados , Dec. 6-11, 1909.J Mr. Cave found that on 
Dec. 8th, at an elevation of some 4,400 feet, the air- 
current formed an angle of 60° with the wind below. 
On Dec. 10th, at 6.10 a.m., at a height of 5,500 feet, 
there was a change in the direction of the wind 
amounting to 50°. When, therefore, the migrant 
bird may appear to have the wind on his shoulder 

* See Encyclopaedia Britannica , Vol. i, p. 267 (“ Aeronautics ”). 

f “ Registering Balloon Ascents at Gloucester,” June 23, 1909, 
by W. Marriott (Meteorological Society’s Papers). 

} Published by the Meteorological Sooiety. 


114 


THE FLIGHT OF BIRDS 


or to be facing a head-wind, he may in reality have 
the velocity of a tail-wind added to his own. We 
know as a fact that migrant birds do fly at a very 
great altitude. In 1880 an American ornithologist, 
Mr. E. D. Scott, at Princetown, New Jersey, used 
an astronomical telescope to watch birds passing 
over the disc of the moon at night. To be in focus 
a bird must be not less than a mile distant; it was 
assumed that he would not fly at a greater altitude 
than 10,000 feet. Knowing the angle made by the 
telescope with the horizon, Mr. Scott was able to 
calculate the lower limit; the birds were flying at a 
height of not less than half a mile.* 

Another observer, Mr. F. M. Chapman,} also in 
New Jersey, made a similar use of an astronomical 
telescope for ornithological purposes, and watched 
262 birds crossing the face of the moon. Of these 
233 were calculated to be flying at a height of not 
less than 1,500 feet. It is remarkable that those 
that were at a low level were flying upward, as if they 
had not yet reached the stratum of air most favour¬ 
able to flight. Some of the birds, as they passed 
over the moon’s face, were silhouetted so clearly that 
Mr. Chapman felt confident that he succeeded in 
identifying the species. 

Mr. F. W. Carpenter, in 1905,} tried a much more 
elaborate plan. Two telescopes, set at some distance 
apart—different distances were tried varying from 
ten to twenty-one feet—were directed upon the moon 
during a night in May and again during an October 

* Bulletin of Nuttall Ornithological Club, Vol. vi. 

t The Auk, 1888, pp. 37-39. 

$ See The Auk , April, 1906. 


PACE AND LAST 


115 


night. The eyepiece of each telescope was crossed 
by hairs that divided the field into “ octants,” and 
each observer had a chart of the moon divided into 
corresponding octants. When a bird appeared in 
the area covered by both telescopes, its course across 
the face of the moon was immediately marked on the 
charts by straight lines, and the hour noted. The 
bird, in fact, had the honour of being treated as a 
star ; the angle at the bird subtended by the line 
between the two observers was calculated; a 
parallax was obtained, and very fairly accurate 
calculations were made. In May no bird was 
observed flying at more than 2,400 feet (less than 
half a mile) above the ground ; the lowest was flying 
at a height of 1,200 feet. In October the birds 
ranged from 5,400 feet (over a mile) down to 1,400 
feet. The calls of birds not far overhead were heard 
frequently during the observations, and Mr. Car¬ 
penter remarks that, probably, most flew lower than 
those observed through the telescopes. I don’t 
understand this, since Mr. Carpenter says that 
objects as near as 1,000 feet were distinctly visible. 
If so, why did the low-flying birds not come into 
view ? 

It is quite possible, then, that migrant birds get 
much assistance from the wind. Though they often 
fly in such a direction that our low-level wind would 
strike upon their shoulder or even blow straight in 
their faces, yet a higher current may be shoving them 
onward, for they fly at an altitude up to which it 
frequently happens that the wind that sweeps over 
the earth’s surface does not extend. Is it not 
possible that Homing Pigeons, though they fly lower 


116 


THE FLIGHT OF BIRDS 


than most migrants, may sometimes get into the 
higher current ? If occasionally they make very 
good times, apparently against the wind, may not 
the explanation be that they have got into a stratum 
of air that was moving in another direction ? Mr. 
Glaisher, as I have pointed out, found that the 
direction of the wind sometimes changed at a height 
of no more than 500 feet. A Homing Pigeon, when 
he is starting, circles upward and takes his bearings, 
looking out for landmarks in the direction of the cot 
for which he is yearning, and it is possible that as 
he rises he may sometimes find a favourable breeze 
though the lower one is adverse. I suggest this, 
since a high authority on Homing Pigeons maintains 
that anticyclonic weather is the important thing, 
not the direction of the wind. Looking through 
the best records, however, I find that the low-level 
wind has been such as to propel the birds and add 
to their velocity. If they find themselves in a 
uniform horizontal current travelling in the direction 
in which they are travelling, there is no reason why 
they should be conscious of the movement of the air, 
unless, indeed, they mark the rapidity with which 
they pass their landmarks on the earth’s surface 
and draw the inference that there is a tail-wind 
adding its velocity to theirs ! They must, of course, 
fly faster than the wind, or the air will give them no 
support. Now, supposing their own efforts give them 
a velocity of 35 miles an hour and the wind has a 
velocity of 25, the two together make up the splendid 
total of 60. At present I say nothing about the 
question whether birds are able to fly with the wind 
when it is blowing a gale. Somehow the belief has 


PACE AND LAST 117 

established itself that a bird would rather face a 
gale than fly before it. 

Endurance, 

I have already to a great extent dealt with the 
question of endurance. It cannot be separated from 
the question of pace when one is discussing the great 
southward flight of the American Golden Plover or 
the voyage of the land birds to their winter home 
in New Zealand, or even the flights of Homing 
Pigeons. In 1892 the bird that won the great race 
of the Manchester Flying Club from Nantes kept up 
a speed of 35| miles per hour for a distance of 430 
miles. He was flying for over 12 hours, during which 
his flight muscles had no rest. I am assuming that he 
did not stop to rest. If he did stop for any length 
of time, his speed while he was on the wing must 
have been much over 35 miles. The wind was 
favourable, from the south-west, and the weather 
was fine, but this does not make the achievement a 
commonplace one. Another bird, when the Preston 
and District Homing Society had their Nantes race, 
flew home—a distance of 441 miles—at a rate of 36 
miles per hour. A man considers 30 miles in a day 
a long walk. What a horse without a rider could 
do I don’t know, but I feel sure it would be some¬ 
thing far short of 400 miles. 

In April, 1909, when I was on my way to Egypt, 
some Herons which were making their spring-migra¬ 
tion flight over the Mediterranean showed astonishing 
endurance. When they first appeared—a flock of 
rather over 20—at 5.30 in the afternoon, they must 
already have flown at least 300 miles from the coast 


118 


THE FLIGHT OF BIRDS 


of Africa. We were at the time just south of the 
Adriatic ; rain came on about ten in the morning, 
and numbers of birds, finding themselves enveloped 
in cloud and unable to see their way, descended to 
the clear air near the surface of the sea and so became 
visible. Throughout the day there were continually 
fresh arrivals. Many accompanied the ship for 
miles, flying round and round her, and in some 
cases settling and resting ; otherwise they remained 
continuously on the wing. When the passengers 
had retired to their berths the Herons were still 
describing circles round the ship. Next morning 
they were gone, and the officers who had been on 
duty on the bridge reported that ten of the number 
had flown round and round till the sun rose and then 
had gone off northward, having probably sighted 
the west end of Crete. In all they must have been 
at least 16 or 17 hours on the wing. 

Migrant birds seem to have altogether exceptional 
power of endurance. Mr. J. L. Bonhote has called 
attention to the remarkable fact that when starting 
for a long voyage they are exceedingly fat, whereas 
they are thin when they reach their destination.* 
They would seem to live on their own fat, as the 
tadpole, when he is becoming a frog, lives by absorb¬ 
ing the fat in his tail. This is a very interesting fact 
which helps us to see how the thing is possible. But, 
in spite of actual or possible discoveries, the lasting 
power of migrant birds must always excite our 
wonder. Supposing that one of them takes no more 
than 130 strokes per minute—a very slow stroke— 
then, if he is on the wing 12 hours, the flight muscles 
* See Ornis, Feb. 1909. 


PACE AND LAST 


119 


contract 93,600 times ! The red, stringy Depressor 
muscle can claim more credit for these marvellous 
flights than any of the others. The heart, the lungs, 
the whole machine, must be very strong and in 
perfect working order. 

Sometimes birds make long flights, requiring great 
endurance, in the course of their day-to-day life. 
Of this I give an astonishing example, which is 
vouched for by two highly competent observers : 
4 4 Another fact that well-nigh struck dumb the 
authors was that Ducks shot at dawn at Daimiel are 
found to be crop-full of rice. Now the nearest rice 
grounds are at Valencia, distant 180 miles ; hence 
these Ducks, not as a migratory effort, but merely 
as incidental to each night’s food supply, have sped 
at least 360 miles between dusk and dawn ”*—and, 
we may add, are probably ready to do it again the 
next day and the next. Such a trifle as 360 miles 
seems to put no strain on the digestive apparatus 
or any part of the organism. The heaviest meal is 
dealt with easily and causes no torpidity. 

Whatever help the wind may give, it would seem 
to be a fair inference from the examples which I 
have quoted that birds surpass other animals in 
vitality. It is not for nothing that their normal 
temperature ranges in the case of some species to 
111° F., and even slightly over. 

* Unexplored Spain , p. 187, by Abel Chapman and Walter Buck- 


CHAPTER X. 


WIND AND FLIGHT. 

RISING—FLIGHT WITH THE WIND—UNDULATING FLIGHT WITH¬ 
OUT MOVEMENT OF THE WINGS-ADVANCE IN A DIRECT LINE 

WITHOUT MOVEMENT OF WING-ADVANCE SIDEWAYS IN A DIRECT 

LINE—SOARING—SOARING IN A HORIZONTAL WIND IMPOSSIBLE. 


No incline, however slight, escapes the notice of 
the bicyclist. He is as sensitive as a spirit-level. 
In the same way there are many birds that detect 
every up-current, even the most local, and make 
use of it, and, no doubt, detect the down-currents 
no less. When they have no serious business on 
hand they will practise manoeuvres in the air, making 
the wind, as far as possible, do the work that would 
otherwise fall on their muscles. A wind with an 
upward trend will often lift them as if they were 
feathers and nothing else, provided they throw 
themselves into the correct attitude and hold their 
wings rigidly extended. Nothing but an upward- 
blowing wind can do the whole work of lifting, but 
a wind that is not of uniform velocity may be of 
assistance. In order to profit by the inequality, 
the bird must pass from a comparatively slow current 
of air into a comparatively rapid one. There is 
hardly a bird possessed of the power of flight that 
has not the skill to turn to account the fact that the 


WIND AND FLIGHT 


121 


wind near the earth’s surface increases in velocity 
with altitude. 

Rising. 

Almost everyone has noticed that birds always 
face the wind when they rise. In the case of big, 
heavy birds this is particularly striking, for they 
will often fly some little distance in the wrong direc¬ 
tion, in a direction in which they certainly do not 
wish to go, in order to get the help of the wind in 
rising. When they have gained some little altitude 
they turn and make for their objective. When a 
steamer disturbs a Gannet floating on the water, if 
there happen to be a fresh breeze blowing from the 
steamer towards him, he will in rising head towards 
the imagined enemy, and not till he has at his 
disposal a few feet of altitude will he turn and make 
off rapidly with the wind behind him. Oyster- 
catchers will do the same thing. Once, when walking 
along the sands south of the Solway Firth, I saw 
hundreds of them in front of me. There was a 
strong breeze blowing from me to them. Hence it 
was much easier for them to make a start if they 
flew towards me till they attained some slight 
elevation. They therefore flew a little way towards 
the disturber of their peace, then turned and settled 
some way off upon the sand. I put them up a good 
many times, and each time they began by flying a 
short distance towards me ; so important is it for 
a bird to get the help of the wind in rising. I once 
saw a Cormorant fly a quarter of a mile or so in the 
wrong direction. He had been feeding with his 
fellows, which had all, after the meal, retired to a 
rock, where they were drying their wings in the usual 


122 


THE FLIGHT OF BIRDS 


Cormorant style. There was a fresh breeze blowing 
from the feeding-place to the rock. As a preliminary, 
therefore, he flew some considerable distance in the 
opposite direction, then turned and joined the others. 

If the wind is to help the bird to rise he must have 
inertia; in other words, he must not drift with it like 
a balloon, but offer resistance. When he faces the 
wind and takes his initial jump, obviously he has 
inertia, for he has just left the earth, which is not 
moving with the wind. Facing the wind he gets it 
to help him. Imagine what would happen if he 
faced in the direction of the wind ; impinging on 
his back it would drive him downwards to earth 
again. But the assistance got from the wind does 
not end with the first jump into the air. The wind 
rapidly increases in velocity with altitude. During 
a terrific blizzard I once saw some Gulls with effort 
making headway against the blast; they succeeded 
only by flying so close to the ground that their wings 
almost touched it. Even when there has been no 
gale blowing, but only a fairly stiff breeze, I have 
noticed that Gulls, heading against the wind, will 
fly as low as possible. Friction reduces the wind’s 
velocity. Some years back some friends and I called 
in the help of an anemometer in order to get more 
definite evidence. On one occasion the anemometer 
recorded a velocity of 770 feet per minute at a 
height of 2 feet from the ground ; at 7J feet the 
velocity rose to 1,000 feet. 

Here are facts of the utmost importance to bird 
and to aviator. We may for the sake of clearness 
divide the air into distinct successive streams, the 
second more rapid than the lowest, and each, as we 


WIND AND FLIGHT 


123 


ascend, more rapid than the one below it, the rate 
of increase growing less as the earth is left farther 
and farther below. As he mounts through each 
successive stream of air the bird has always inertia; 
he is never, like a balloon, the toy of the breeze. And 
this, not only because he keeps plying his wings, but 
because he is always emerging from a slower stream 
of air into a more rapid one. Consequently, quite 
apart from his vigorous wing-strokes, he offers 
resistance to the wind ; he has, in fact, the inertia 
that is indispensable. The force of the horizontal 
wind is broken up into two forces, one of which tends 
to lift him. The Lark, that past-master in the art 
of upward flight, always gets the wind to lighten 
the work of his wings, even up to considerable alti¬ 
tudes. Sometimes, when the wind fails, he will sweep 
vigorously round in a wide circle and make the 
velocity due to his own efforts to some extent take 
the place of a wind. Big and small alike, all birds 
are glad to have the help of the wind. The muscles 
that lift their wings are by no means strong, but as 
soon as they have got some way on the rush through 
the air does the work of lifting. A big, heavy 
Elevator muscle would, therefore, be a useless 
encumbrance during horizontal flight; it is best to 
put all the strength into the Depressor. Not only 
is the Elevator small, but, as I have shown, it is of 
inferior quality ; it has not much last. All the more 
reason, therefore, to use artifice in order to economize 
effort in rising ; or, to put it more correctly perhaps, 
birds have not developed high-class Elevator muscles 
since their skill rendered them unnecessary. Big 
birds require the help of the wind to lift them at the 


124 


THE FLIGHT OF BIRDS 


start much more than small birds. I have twice 
seen Snipe, if not, strictly speaking, rise, yet begin 
horizontal flight with the wind behind them. This 
shows what a good start a bird that has strong and 
fairly long legs gets from his first jump forward and 
upward from the ground. Things are very different 
with a Swift or a Puffin, owing to the shortness and 
feebleness of their legs, or such a bird as the Condor, 
that has no room when he is starting for the full 
sweep of his great wings. It is bad starters like 
these that are in difficulties when there is no wind to 
help them.* 

Flight with the Wind. 

I have already pointed out that Homing Pigeons 
make their best “ times ” when there is a fair breeze 
blowing in the direction of their flight. And for 
all birds that fly, not only for Pigeons, there is 
every reason to believe that a tail-wind means rapid 
travelling. But what if the wind be a gale ? Some¬ 
how or other an idea has grown up that birds cannot 
fly, or at least do not like flying, with a gale blowing 
from behind them. It is curious how hard it is to 
get actual positive evidence for or against. Can we 
find competent observers who have seen birds racing 
before a regular gale ? And who has seen anything 
from which we may reasonably infer that a gale from 
behind is an abomination to them ? Some people 
are satisfied that they have settled this question by 
remarking that “ it would, of course, ruffle up their 
feathers, and it is an undoubted fact that birds 

* If there is anything of a wind, a bird, if flying with it, always 
wheels round and faces it when he alights, and the prudent 
aviator follows his example. 




125 


WIND AND FLIGHT 

dislike this.” By way of answer we may say that 
an undoubted fact is no better than blank cartridge 
if it does not apply to the particular case. It is true, 
no doubt, that whereas a horse or a cow will always 
turn tail to a strong breeze, a bird always stands 
facing it. I have just seen some forty Starlings on 
a dead poplar tree. All, to a bird, were facing the 
breeze. Gulls or Oyster-catchers may often be seen 
by the seashore all facing to the front like soldiers 
when the word “ Eyes front! ” is given. The 
nictitating membrane protects their eyes from the 
wind, and there is no tumbling of their neat plumage. 
But would such a thing be likely to happen during 
flight from whatever direction the wind was blowing ? 
A bird must have some velocity of his own ; he 
cannot be like a leaf carried by the wind ; he is too 
heavy for such a method of travelling, and, were he 
to try it, he would soon descend precipitately to 
earth. But though he necessarily has velocity of 
his own added to that of the gale, it might seem 
possible that a terrific tail-wind might occasionally 
cause difficulty. Though the bird’s body outpaces 
the gale, yet there may be moments when the gale 
overtakes his wings. When the down-stroke is over, 
the wings are moved backward as well as upward 
preparatory to the next stroke, though in long¬ 
distance horizontal flight the forward and backward 
movement is not very great. The velocity of the 
wind, too, varies from moment to moment, and 
Professor Langley showed that the greater the 
velocity the greater is the irregularity. Even a 
steady wind proves to be gusty when properly tested. 
If, then, the wings happened to be moving backward 


126 


THE FLIGHT OF BIRDS 


just at the moment of a terrific gust, when the gale 
was outdoing itself, they might possibly be unable 
to move backward as they should. Certainly this is 
imaginable, though I cannot help thinking that the 
bird, though flustered for a moment, would cope with 
the difficulty. It must be remembered that it is 
only at slight altitudes above the earth’s surface 
that the wind is so capricious and irregular ; the 
obstructions it meets with there make it a broken, 
boisterous torrent. The migrant bird flying high 
above us is in a calmer stream, however rapid its 
onward sweep may be. What we want is positive 
evidence. Being in difficulties some years ago about 
this question, I was delighted when I came across a 
paper read before a learned society in Germany on 
this subject. The writer maintained that birds did 
often fly with a gale blowing from behind them, but 
that under these circumstances their flight is so rapid 
that we do not see them ! And the learned society 
printed his paper ! Commander Lynes has recorded 
an interesting observation that might seem to throw 
light upon the matter. A migrating Swallow was 
flying in the wrong direction, northward, when he 
should have been flying southward.* Apparently 
a gale sprang up, a gale from behind, and the Swallow 
in consequence, as it appeared, turned and was flying 
against the wind, as if intending to return to the place 
from which it had set out. It must not be forgotten, 
however, that migration, as a rule, proceeds at a 
great height, and that there the wind may not be 
blowing from the same quarter as it is at our lower 
level. At Alderney some years back I saw what I 
* British Birds , Vol. in, p. 141. 


WIND AND FLIGHT 


127 


attributed to unwillingness on the part of small 
shore birds, Ringed Plovers and Dunlins, to fly with 
a gale behind them. It was a real gale. I could with 
difficulty make any headway when I faced it and 
tried to walk. The small birds kept flying from one 
patch of sand to another (the patches were scattered 
among the rocks). They never headed towards what 
seemed to be their objective, if, in order to reach it, 
they would have had to fly with the wind behind 
them. They would first face the wind and gain some 
little altitude, and then turning, so that they faced 
at about a right angle to it, let themselves be swept 
to the patch to which, apparently, they wished to 
go. Of this I saw a good many instances. But we 
cannot build much on such observations, and having 
since that time had occasional chances of watching 
similar manoeuvres in rather less violent, but still 
very strong winds, I am unable to draw the conclu¬ 
sion that I then did. 

Birds are very fond of playing when upon the 
wing, and a good stiff breeze or a gale gives them 
fine opportunities. When nestlings they have very 
little chance of playing ; they are too crowded in 
the nest, and in the case of many species restlessness 
would end in a fatal fall. But as soon as they have 
the use of their wings, the representatives of many 
species are never tired of aerial sport. No doubt 
this in a sense is practice. The Swallow improves 
in agility, and so is better able to catch gnats when 
he wishes to catch them. For the time he is not 
definitely on the hunt, but is simply enjoying the 
evolutions that the wind makes possible. If any 
one doubts this, let him watch Gulls circling by the 


128 


THE FLIGHT OF BIRDS 


hour together above a cliff that gives the wind an 
upward incline. There are no fish to be caught 
there, nor anything material to be got by the per¬ 
formance. They are describing spirals high in air, 
and by their skill they get the wind to relieve them 
of all hard work. No wonder they enjoy it. To 
return to the Swallows, whose evolutions are 
different. When there is a fairly strong breeze 
blowing, the first thing is with its help to attain some 
altitude ; they face the wind and it helps to lift 
them. Then, taking advantage of their position, 
they enjoy a variety of downward glides. Often they 
will face the wind and glide downward sideways, their 
course thus crossing the stream of the wind. If 
the wind has an upward trend this may be done 
without loss of elevation, as I hope to show later on. 
They can, of course, glide downward in the teeth 
of the wind. Sometimes they will glide sideways 
in the direction of the wind. In this case there are 
often two simultaneous movements. The bird glides 
downward, head leading, across the stream of the 
wind, but the gale sweeps him along sideways with 
such velocity that the other movement is obscured. 
I think this is the explanation of what I saw at 
Alderney. The birds were practising a sporting 
manoeuvre; gaining a little elevation and then 
letting themselves be carried sideways by the gale. 

Some aerial evolutions that I have just been 
watching—Swallows and one Swift were the per¬ 
formers—included no straight a-head flight before 
the wind: it was a strong wind, though not a gale. 
But I do not infer a positive dislike of such a thing. 
The manoeuvres consist of varieties of tobogganing, 


WIND AND FLIGHT 129 

which familiar sport includes for human beings one 
thing that is not enjoyable, viz., the lugging of the 
toboggan to the top of the slope again. This part 
of the performance is laborious for the bird also, 
unless he can get help from the wind. This help 
he gets when he faces it. When he has risen high 
enough, he can plane down in whatever direction 
he may choose to go. If he chooses to go with the 
wind, he generally chooses the sideways method 
that I have described. If he were to go head leading 
and if his line of descent made only a slight angle 
with the horizontal, there would probably be a 
ruffling of his plumage, the thing that he abhors. It 
could not happen were he to take vigorous strokes, 
though the backward movement of the wing for the 
next down-stroke might imaginably, as I have shown, 
be a difficulty. During the Swallow manoeuvres 
some Starlings flew past with the wind directly 
behind, at a considerable height, too, where the wind 
must have been even stronger. Their pace was no 
mean one, and they seemed to be suffering no kind 
of inconvenience. If the wind had freshened to a 
gale, would it have been no longer a help but a 
hindrance ? 

Since writing the above I have had a chance of 
watching Rooks and Starlings flying in so strong a 
wind that it might fairly be called a gale. The 
Rooks made use of up-currents and rose to a great 
height. Some of them flew before the wind, putting 
in vigorous strokes occasionally in order to outpace 
it. They seemed not to be in any way inconveni¬ 
enced by it. Though Tennyson speaks of Rooks on 
a wild, windy day being “ blown about the skies,” 


130 


THE FLIGHT OF BIRDS 

these Rooks never for more than an occasional half- 
second lost command of their movements. The 
Starlings were no less at their ease, so that, to my 
thinking, this curious little problem of flight with a 
gale blowing from behind is settled. Birds are 
capable of such flight, and much enjoy it. But to 
migrate in such wild weather would be a different 
matter. It is no wonder that they do not choose 
a dark, stormy night for a long oversea voyage. 

Undulating Flight without Movement of 
the Wings. 

At one time I believed that this was possible when 
the wind was blowing horizontally, but I now feel 
sure that an upward trend is necessary. Take an 
example that is often to be seen. The wind is 
blowing at something like a right angle to the course 
of a steamer, and the Gulls, which are following to 
pick up any scraps that may be thrown into the sea, 
soon recognise that there is a chance of saving their 
muscles. There is an up-current of air on either 
side of the ship, for the wind is, so to speak, bent 
upward some little distance before it reaches the 
obstruction. To start with, then, the Gull obtains 
some slight elevation by means of a few strokes, then 
he glides down a gentle incline in the direction in 
which the ship is travelling. When close to the 
water he turns and faces the wind, which, having 
an upward trend, lifts him to the level from which 
he has just descended. He then glides downward 
again, and so the process goes merrily on. 

Were the wind a horizontal one it would, no doubt, 
help the Gull to rise, but it would not make all 


WIND AND FLIGHT 


131 


exertion on the bird’s part unnecessary, any more 
than it does in the case of the Snipe that faces the 
wind in rising. 

There is no reason why a bird should not advance 
in the same way in the teeth of a wind when the 
conditions are favourable, and occasionally Gulls 
may be seen employing this method behind a steamer 
when the wind is directly or almost directly ahead. 
As the vessel moves onward, there is a down-current 
behind the stern, since the air rushes down to fill 
the space just vacated by the advancing ship. The 
down-rush of air strikes the water and, rebounding 



A W 


Fig. 25. 

Flight at right angles to the wind, with motionless wings. W : 
the direction of the wind. 

from it, forms an up-current a little way farther 
back. Here, then, is an up-current extending only 
over a very small area of water, but, since it moves 
on with the steamer, it opens up great possibilities. 
To start with, imagine the Gull flying a little in rear 
of the up-current. He glides swiftly downward and 
onward, and when near the water finds himself in 
the convenient up-draught, which lifts him to his 
former level, so that he is able again to glide down¬ 
ward and make much headway. At the end of his 



132 


THE FLIGHT OF BIRDS 


glide he generally hits off the up-current, but not in¬ 
frequently he fails and has to put in a few strokes. 
Small pieces of paper thrown over the stern show that 
the up-current is not always at the same distance 
behind the ship. Hence this method of advance, 
though a very gay and lively one, has, whenever I 
have observed it, been lacking in precision. How¬ 
ever, when the conditions are not perfect, the Gull 
may be trusted to make the best of a bad job. I 
have just been watching some Gulls that were 
following a small steamer. The wind was blowing 
nearly at a right angle to the vessel’s course. There 
was an up-current available, but apparently an 
unsatisfactory one, for there was never an advance, 
except for a few moments, without very distinct 
wing-beats. The Gulls faced the wind and, while 
almost uninterruptedly beating with their wings, 
advanced nearly sideways, right wing leading. Had 
there been no up-current, such a style of flight would, 
I believe, have been impossible. They must have 
been inclining their bodies slightly downwards, from 
the left side to the right, so as to induce movement 
in the direction desired. I am quite aware that 
birds, when they wish to advance at right angles 
to an ordinary horizontal wind, make a half-turn 
towards it, so as not to be swept out of their course. 
But here was an instance of a much more complete 
turn and an advance almost sideways. 

Clever as the Gull is at such methods of advance, 
the Shearwater, to my mind, is a yet more perfect 
master of the art. He does not require any steamer 
to help him. If only there are waves and a wind, 
he has all the conditions that he wants. He keeps 


WIND AND FLIGHT 


133 


gliding downward and onward till he is almost 
touching the water, then suddenly he faces the wind, 
showing his white under-surface, and is lifted a few 
feet, then he glides downward and onward again, 
again faces to right or left, and is again lifted 
without any sign of effort on his part. It is 
a weird performance, more impressive than the 
Gull’s. 

There must be up-currents when required, other¬ 
wise the Shearwater must be superior to physical 
laws, and such a superiority we cannot concede. 
The waves give to the wind the upward incline that 
is wanted ; even a small obstruction will cause a 
very considerable deflection (see p. 134). Advancing 
at something like a right angle to the wind, he feels 
an up-current as he is gliding downward and onward 
and at once turns and faces it; thus he gains altitude 
and can begin another onward glide. And so he is 
able in most methodical style to cover large tracts 
of sea. Gannets may be seen employing the same 
method in British seas, and, no doubt, Shearwaters 
also, though I have seen them chiefly in the Mediter¬ 
ranean, where they are common. As a rule I have 
seen them advancing in this style at a considerable 
angle to the wind, but they sometimes employ the 
same method for an advance in the teeth of the 
wind. There is no reason why the Shearwater and the 
Gannet, having the waves and the resulting up- 
currents to assist them, should not do what Gulls 
do behind a steamer when there is a wind blowing 
across the vessel’s course—I am speaking of the 
clever performance described above (see fig. 25, 
p. 131). The Shearwater and the Gannet have at 


134 


THE FLIGHT OF BIRDS 


their disposal up-currents at short intervals, and 
these are just as serviceable to them as the travelling 
up-current behind a steamer is to the Gull. 

Before I go further it will be best to mention some 
experiments I once made with a vane which, instead 
of swinging round to show from what point of the 
compass the wind was blowing, worked vertically 
up and down and detected up-currents and down- 
currents of air. For its large arm it had a thin piece 
of deal, one foot long by six inches broad, and this 
was exactly balanced by a lump of lead at the end 
of the shorter arm. There happened to be at New 
Romney, where I made these experiments, a number 
of banks forming barriers of a very convenient 
height. While standing on a bank only two feet 
high, its tripod lifting it four feet above the bank, 
the vane pointed decidedly upwards. Five yards to 
leeward of a bank six feet high it indicated that the 
wind blew downwards, making a large angle with the 
horizon ; there was but rarely an upward gust. Ten 
yards to leeward of the bank the direction was still 
mainly downward, but with not unfrequent upward 
movements. At twenty and at thirty yards’ dis¬ 
tance the wind came in wild gusts, as often upward 
as downward. On the windward side of the bank 
the results were no less remarkable. Twelve yards 
to windward the vane was not quite steady, but on 
the whole horizontal. At a distance of six yards 
there were occasional upward swings ; at four yards’ 
distance there was a decided upward tendency, and 
this though the bank itself presented only a very 
gentle incline. These facts set one thinking. If a 
bank only six feet high is capable of so much, what 




WIND AND FLIGHT 


135 


splendid up-currents must mountain ridges put at 
the service of a soaring Eagle ! 

Advance in a Direct Line Without Movement of Wing. 

There is a feat perhaps more striking than any of 
those already described, a feat which, nevertheless, 
Gulls often achieve. A steamer is advancing against 
a fairly strong wind which, if not absolutely a head¬ 
wind, strikes the vessel at an acute angle. There 
results a steady up-current over the stern of the 
vessel, or slightly to one side or the other of the stern. 
Poised on this up-current the Gulls hang in mid-air, 
their wings held rigidly expanded. Only very slight 
wing-movements, evidently for purposes of balance, 
can be detected. Standing on the deck and watching 
these Gulls one is irresistibly reminded of the poising 
of the Kestrel high in air with wings held motionless, 
when he finds a wind that is all that he could wish. 
It is sometimes easy to forget that, unlike the 
Kestrel, they do not remain in one spot, but that 
all the while they are moving onward and, in fact, 
keeping pace with the steamer. The Gulls, like the 
Kestrel, are poising on an up-current of air, but 
they give their bodies a rather different incline, with 
the result that they keep travelling forward. The 
diagram will explain this. The general incline of 
their body and wing surfaces is slightly downwards. 
Hence the upward-streaming wind not only main¬ 
tains them in air or lifts them higher, but, acting at 
a right angle, also drives them forward. Imagine 
a bird with his body sloped much more steeply 
downward. Obviously, the wind would then give 
him a shove forward. What the Gull does is like, 


136 


THE FLIGHT OF BIRDS 


but with a difference, to a simple downward glide in 
still air. In the downward glide, the bird or aviator 
has to obtain a certain downward velocity by the 
help of gravity before the air will give him support. 
In the method of advance I have been describing no 
downward velocity is required, since the air has an 
upward trend and resists the pull of gravity. Some¬ 
times this method of making headway without any 
motion of the wings may be seen in mountainous 
countries, where it is sometimes practised by 
feathered experts capable of nobler achievements 


B 


Fig. 26. 

Illustrating advance, with wings held rigid, in the teeth of the 
wind, the wind having an upward trend. B-D, the body of the 
bird sloped slightly downward. The arrow represents the wind. 
Its force acting along F-M is broken up into two forces 
represented by S-M and R-M. 

even than the Gull. In Algeria I once saw two 
Eagles sail straight ahead against the wind for about 
a mile and a half without moving their wings till they 
reached a high mountain ridge, blowing over which 
the wind had got an upward trend. Having done 
their mile and a half, they came back with the wind, 
beating with their wings, and then repeated their 




PLATE XVI. 



A and B : Gulls following steamer, with motionless wings. Photo¬ 
graphed from the stern of the ship. In A they are almost overhead. 
The fact that there are so many with their wings horizontal shows that 
it is not a case of ordinary flight. (See Chap. X.) 


To face p. 136.! 


































































WIND AND FLIGHT 


137 

majestic performance, but this time at a higher 
level. It is not often, I believe, that there is a 
steady upward-trending wind extending over so 
long a reach. In the Alps I recently saw an Eagle 
perform a similar feat, but not on so grand a scale. 

It is clearly possible for a bird to advance in this 
way facing with the wind instead of against it. The 
wind acts at right angles to the bird’s expanse of 
wing and body, and, except for friction, it will not 
matter whether it blows from in front or behind. 
If it has a sufficient upward incline, it will not get 
between the feathers and ruffle them. But, as a 
fact, one very seldom sees a bird advancing in this 
way. Still I have occasionally met with examples, 
though I doubt whether the wind comes quite 
directly from behind ; the bird turns himself just a 
little sideways. Circumstances are favourable when 
the wind blowing at a very slight angle against a cliff 
is deflected upward. The Gull then advances with 
the wind, head leading, wings motionless. But as 
I have said, there is, probably, always a very slight 
sideways turn made, so that the body from back to 
tail is not quite in line with the course of the wind. 
Is this to avoid a disarrangement of feathers—that 
bothersome question ? Whatever the explanation 
of this very slight deflection, and though this method 
of utilising an up-current is not so common as others, 
yet it is important that it should obtain recognition. 
It will help us to understand soaring when we come 
to investigate that difficult subject. 

The wonderful flight of the Albatross, his wings 
with their spread of twelve feet or more held motion¬ 
less, I cannot undertake to describe, as I have never 


138 


THE FLIGHT OF BIRDS 


had the luck to see it. Sometimes he seems to hang 
motionless over the stern of the steamer in the 
style with which Gulls have made us familiar. As I 
understand the story, the Albatross in Coleridge’s 
famous poem was poised on an up-current above 
the ship’s stern, presenting a big, steady target 
impossible to miss, when the Ancient Mariner was 
mean enough to shoot the unsuspecting bird. Some¬ 
times, I am told, the Albatross sweeps majestically 
downward to a point some way off from the ship, 
his wings all the while outstretched. It seems that 
he must be practising the same kind of manoeuvre 
that Gannets or Shearwaters practise, in their com¬ 
paratively humble way, when they advance without 
a motion of their wings at right angles to the wind, 
or, occasionally, with the wind or against it. Accord¬ 
ing to accounts, the Albatross takes downward 
sweeps on a gigantic scale. Those who describe it 
say that his evolutions carry him far away from the 
vessel. How, then, are we to provide him with an 
up-current that will lift him without his having to 
move his giant wings ? What but the waves can 
deflect the wind for him, when he has planed down 
to regions beyond the steamer’s sphere of influence ? 
The Albatross is as completely subject to physical 
laws as any other bird, or as a man of twenty stone 
weight, and we may depend upon it that, if he is to 
rise in the air, it must either be by means of the 
contraction of muscles and powerful wing-strokes 
or else by the help of an ascending current of air. 
Those whose good fortune it is to see this noble bird 
at his play should watch him carefully and note all 
the conditions, instead of merely gazing, as many 


139 


WIND AND FLIGHT 

seem to do, in open-mouthed astonishment. There 
is one question that, I believe, has been but little 
investigated. In tropical seas there may be up- 
currents rising from the heated surface, just as there 
are from sun-scorched plains even in much higher 
latitudes, up-currents sufficient to serve the turn 
of the Albatross. But his evolutions are to be seen 
in Antipodean regions where no such heating is 
likely to take place. Myself I have little doubt that 
the Albatross’s art is only that of the Shearwater, 
though, owing to the great artist’s enormous spread 
of wing, the effect produced is much grander. A 
writer quoted in Flight (Feb. 3rd, 1912) describes 
it thus : “ The flight is generally near the water, 
often close to it. You lose sight of the bird as he 
disappears between the waves and catch him again 
as he rises over the crest. . . . He alters merely the 
angle at which his wings are inclined.” Why, it is 
just in this style that a Shearwater sweeps down¬ 
ward, and then, by the help of a wave and the 
resultant up-current of air, regains all the altitude 
he has lost! 

Advance Sideways in a Direct Line. 

To return to the Gull, a more commonplace, yet 
intensely interesting subject. Often, when he wishes 
to advance at right angles to the wind, he faces it 
and travels with motionless wings sideways, or, 
more correctly, almost sideways. As if thinking of 
his objective, he inclines head and body very slightly 
towards it. We are, of course, presupposing an 
upward-trending wind ; the Gull is poised upon it. 
If the left wing leads, then the wind must be blowing 


140 


THE FLIGHT OF BIRDS 


not exactly straight in the bird’s face, but very 
slightly from the right. A very little shove is enough, 
as the air offers no great resistance to so well-built 
an aeroplane travelling at no more, at any rate, than 
twenty miles an hour, and generally slower. A 
man-made aeroplane is bound to move at a great 
pace or the air will not support it. But in the case 
we are considering it is the wind that must have the 



Gull gliding sideways, the left wing leading. The wind 
(represented by the arrow to the left) is deflected upward 
by the cliff. 

pace. Poised upon the strong, upward stream, the 
Gull goes gently on his way. Some years ago, when 
I was at Port Erin, in the Isle of Man, it was inter¬ 
esting to see the Gulls returning in the evening to 
the little island called the Calf, at the southern 














WIND AND FLIGHT 


141 


extremity of Man. The wind blowing from the west 
struck against the cliffs and was deflected upward. 
The Gulls, as they always do, saw their chance ; 
here was a fine, effort-saving up-draught. Flying 
to the base of the cliff, they were lifted to the top 
and far above it. They would then turn and face 
the wind, and, with the left wing leading, return to 
their night quarters, their heads being inclined just 
a little towards the south. Occasionally Gulls adopt 
this method of travelling when the wind blows 
almost at right angles to the course of a steamer. 
They will hang over it and keep pace with it, their 
wings pointing to bows and stern. The slight adjust¬ 
ments that they have to make for balancing purposes 
are unceasing, but they are easily distinguishable 
from the strong wing-strokes of ordinary flight. 

The two methods—advance in the teeth of the 
wind and advance with one wing leading—pass into 
one another. Obviously so, since, to get support 
from an upward trending wind, the Gull either 
faces it or much less often turns his tail towards 
it. Hence the necessity of progression sideways 
when he wishes to travel in a straight line at a 
right angle to the wind. When he travels at a half 
right angle to it, the line of advance will bisect the 
angle between the axis of his body and his wing. 
To put it less mathematically, he will be half-side 
face towards his line of advance. 

Soaring. 

Perhaps a definition of soaring may be useful. 
The word is used to describe the spiral ascent of a 
bird in the air, effected without taking any strokes 


142 


THE FLIGHT OF BIRDS 


with his wings. He gets the wind to lift him, and 
as he rises he circles, or, more correctly, describes 
a spiral or helix. It is a marvellous performance. 
Had we not an unlimited capacity for getting used 
to anything, we should be lost in wonder whenever 
we see this splendid achievement. 

Many of the larger birds are proficient at it— 
Eagles, Vultures, Pelicans, Storks, Falcons, Kites, 
Buzzards, Ravens, Gulls and others, all, even the 
smallest of them, possessed of wings that have a 
very considerable area, and are very different in 
outline from the long, narrow wings of the Tern or 
the Swift. The Gull’s wing is less definitely a soaring 
wing than the others mentioned ; adapted both for 
soaring and long-distance flapping flight, it is a 
compromise between the broad and the narrow.* 

Evidently breadth, and not only length, is im¬ 
portant in soaring, and the great primary feathers 
spread out, leaving very noticeable gaps between 
them. Probably this gives steadiness, preventing 
a too sudden escape of air from under one wing. If 
a bird is watched through a field-glass or telescope, 
the upward bending of these great feathers by the 
force of the wind is sometimes quite noticeable, the 
first primary being often bent considerably more 
than the others (see the Frontispiece). 

It is a slow, sedate movement, this circling high in 
air. Mr. S. E. Peal,*)’ who used to gaze with wonder 
at the circling of the Adjutants, a kind of Stork noted 
for their soaring, over the plain of Upper Assam, 
held, if I remember rightly, that the birds slept as 

* See PI. xiv. 

t See Nature, Nov. 4, 1880 ; Sept. 26, 1889 ; May 21, 1891. 


143 


WIND AND FLIGHT 

they described their sedate spirals. A bird is cer¬ 
tainly capable of a great deal during sleep. When 
he sleeps standing on one leg, he is perpetually 
making small adjustments in order to maintain his 
balance. When a Duck sleeps floating on a pond 
with one leg tucked up, he will keep the other 
paddling, so that he may move in a circle and not be 
driven by the wind into the bank, where a hungry 
stoat may be waiting for him. But to sleep while 
soaring is an altogether different matter. The 
soaring bird has not only to make perpetual adjust¬ 
ments, but also to feel the pulse of the wind, to be 
alive to every gust and find out what adjustments 
have to be made. But the fact that so good an 
observer could hold this theory shows how sedate 
the movement is. Though the pace may vary, there 
is not a rapid sweep down a gentle incline in one-half 
of a circle, then, in the other half, when the bird has 
wheeled round, a slow advance with much gain 
of altitude : nothing corresponding to the gallop of 
the four-in-hand down the last fifty or hundred 
yards of a hill, in order that the coach’s momentum 
may carry it some way up the hill that is immediately 
in prospect. The circling is slow, sedate, and appar¬ 
ently perfectly comfortable, and sometimes certainly 
the bird keeps rising through a whole turn of the spiral. 
He does not sweep downward in one part, then turn 
and gain elevation. If all goes well, if the wind is all 
that is required, there is no loss of altitude from 
beginning to end of the turn ; there may be a gain 
throughout. 

The birds that soar are all of considerable size 
Small birds, however expert in flying, are, apparently, 


144 


THE FLIGHT OF BIRDS 


unable to get the wind to lift them. And yet, rela¬ 
tively to their bulk and weight, their wings are very 
decidedly larger in area than those of big birds. The 
explanation is, I believe, that, though the wing of 
the small bird is relatively the larger, yet, actually, 
it presents too small an expanse for the purpose of 
soaring. The wind, instead of giving the required 
support, escapes at the edges. In ordinary flight, 
when the beating wings move with enormous 
rapidity, their small area does not tell against them 
as it does when the bird merely remains passive and 
waits for the wind to strike it. In fact the velocity 
of the wing’s movement during the down-stroke is 
distinctly greater than the velocity of the wind that 
supports the soaring bird. Even a Heron or a Crow 
in leisurely flight takes not less than 120 strokes 
per minute, the Pigeon, according to Professor Marey, 
480, the Duck 540, and the Sparrow 780. This 
means that the farther part of the wing moves with 
astonishing rapidity. The big soaring bird, more¬ 
over, has a large cup-like concavity near the base of 
the wing, which must hold the wind and so give 
much support. Even the Gannet’s wings, narrow 
and elegant as they are, have near the body a deep 
hollow that serves to catch and utilise any up-current 
that offers. 

Soaring always goes on at a considerable altitude. 
In mountainous countries there are frequent oppor¬ 
tunities of seeing it, and with luck one may occa¬ 
sionally get near to the scene of the performance. 
In Spain I once climbed to the top of a high cliff on 
which was a Vulture rookery, some fifteen nests of 
the Griffon Vulture, and saw the great birds circling 


WIND AND FLIGHT 


145 


round a little below me. In hot countries it takes 
place over wide plains where there are no hills 
near. I have never seen or heard of it under such 
conditions in cold, northern regions. Nor have I seen 
any attempt at it over the sea. Gulls are no mean 
performers at soaring, and they may frequently be 
seen circling in fine style over cliffs. The fact that 
they do not soar out at sea, and that they frequently 
do where there are cliffs of any height, suggests the 
secret of the whole thing. The bird when soaring 
is lifted and maintained by an upward-trending 
wind. At sea, in our northern latitudes, there are no 
up-currents, or none of strength sufficient to make 
soaring possible. Sir Hiram Maxim is able to detect 
up-currents and down-currents of air that leave 
smooth or ruffle our comparatively cold northern 
seas.* It would be folly to deny their existence 
without very definite evidence. It is very difficult, 
however, to believe that they are currents of much 
strength. The water far away from the Tropics 
does not get heated sufficiently to cause a rapid 
upward movement of air. Were the up-currents 
which Sir Hiram Maxim has detected of any force 
and lifting power, would not the Gull and the Shear¬ 
water, quick as they are to avail themselves of any 
little up-draught due to steamer or to waves, hasten 
to make use of them ? I am quite aware that I am 
here guilty, technically, of a petitio principii. I 
wish to show that soaring depends on up-currents 
of air, and I use the fact that Gulls do not soar at sea 
—at any rate not over our northern seas—as evidence 
that there are no strong up-currents. The reader 
* Artificial and Natural Flight , p. 16. 

L 


146 


THE FLIGHT OF BIRDS 


must take the argument for what it is worth. But 
there is direct evidence. Aviators are now exploring 
the air just as birds for ages past have done, and I 
read in an article on military aviation* that remous 
(up and down currents of air) “ are seldom met with 
when flying over a uniform surface such as the sea.” 
In hot countries the air is heated by contact with the 
sun-baked soil. It ascends and when, at some 
height, it gets chilled by contact with colder air, it 
forms cumulus clouds. It is found that the rate at 
which balloons ascend varies much when there are 
clouds of this kind about, and that is good evidence 
that the air in places is streaming upward.t At a 
low level there are no distinct upward and downward 
streams, but at some height above the plains such 
streams begin to form and the great cumulus clouds 
tell us in what parts the movement is upward. One 
day in the Nile delta, a very hot day with a blazing 
sun, I was watching the Kites soaring. They were 
wheeling and wheeling round and not a motion of 
wing was to be seen. Suddenly a cloud obscured 
the sun, and very soon all the Kites began beating 
with their wings, and descended to a lower level. 
It may be maintained that they do not care to soar 
unless it is bright and sunny, and that they gave it 
up because, there being no longer any sunshine, they 
had no pleasure in continuing. Still I have seen 
Ravens soaring over hills in cloudy weather, and 
one day in the Alps, when snow or sleet fell at 
intervals and the wind was raw and nasty, I saw a 

* By Captain Brooke-Popham, Army Review, Jan. 1912, p. 88. 

t See “ Methods for Observing Pilot Balloons,” by C. J. P. 
Cave, Journal of the Royal Meteorological Society , Jan., 1910. 


WIND AND FLIGHT 147 

Kestrel hovering with motionless wings. The up- 
currents over sun-heated plains seem to have force 
up to an enormous height. In Egypt I once watched 
through my telescope a flock of Storks, ascending 
with wings held rigid till they looked mere specks. 
I wondered how they steered clear of one another, 
there were so many describing mazy, intersecting 
circles. It was just noon, and the day was decidedly 
hot. 

Of course there may be among mountains upward 
currents that are in origin similar to those found 
over level plains. The extraordinary heat of the 
summer of 1911 has been well calculated to produce 
such currents in unusual places. I have a letter 
from Mr. R. C. Gilson that gives most striking 
evidence of this. “ Lying on my back the other 
day,” he writes (the letter is dated Sept. 16th, 1911, 
“ on the summit of the Murren Schildhorn, a flat- 
topped eminence (about 10,000 feet) with pretty 
steep sides—at all events two opposite sides are 
steep, the mountain is ridge-shaped—I saw a piece 
of paper carried up by the wind, and having no 
tendency to descend, but the reverse. I then noticed 
another much higher up, then others, an apparently 
indefinite series (the mountain is frequented by 
untidy tourists), of which the farthest that I could 
see were mere silvery specks in the sunshine. How 
high they were it is impossible to say, but I guess 
not far off 1,000 feet above the hill. The weather 
was anticyclonic, almost windless. Presumably the 
elevating forces were convection currents from the 
sun-warmed mountain-side. Where I lay I could 
hardly detect a breeze, but there was always a very 


148 


THE FLIGHT OF BIRDS 


slight one perceptible if one approached close to the 
edge of the hill.” 

What is going on in the air high aloft when soaring 
takes place over a dead level I imagine to be this. 
There is a wind sweeping over the plain, and at the 
outset it is horizontal. Coming into contact with 
an up-current from the heated surface below, it is 
deflected upward. The soaring bird, then, gets 
support not only from the ascending column of air, 
but from the wind to which the ascending column 
gives an upward trend. Were the heated air 
mounting from below the sole support, birds might 
soar in an almost dead calm, and that is a thing 
which all observers agree does not take place. They 
first ascend to some height—two or three hundred 
feet—by beating with their wings, and then the 
performance begins. 

The reason of the spiral movement is not, I believe, 
far to seek. There are over the plain regions of 
ascending and regions of descending air. It is 
essential that the bird should not pass beyond the 
boundaries of the upward stream that maintains 
and lifts him. Had the wind over the whole extent 
of the plain an upward incline, then the Kites and 
the rest might soar, if the term will stand this strain¬ 
ing of its use, in a straight line like the Eagles, whose 
majestic advance, without deviation to right or left, 
I have already described ; or, to take an example 
more commonly seen, like the Gull that follows a 
steamer, poised on an up-draught over the stern. 
The up-and-down currents on which birds depend 
for soaring are sometimes very formidable to an 
aviator, who in a few seconds may pass through an 



WIND AND FLIGHT 


149 


ascending column of air and suddenly find himself 
in a descending one. Captain Brooke-Popham, 
whom I have already quoted, says that it is probable 
that these remous seldom exceed 100 feet in width.* 
In order to understand soaring, which often takes 
place at great heights where it is impossible, at any 
rate for anyone who is not an aviator, to investigate 
air currents, it is well to consider whether it is in 
principle at all different from what we may see taking 
place close at hand where investigation is easy. 
Gulls, as we know, have no difficulty, when the wind 
has an upward trend, in poising upon it and making 
headway against it. We have also seen that, the 
upward trend being there, they will occasionally 
advance, wings motionless, with the wind behind 
or almost behind them. When a soaring Eagle 
wheels round and circles, he does high aloft these 
two things that the Gull does near to the earth. He, 
of course, does something more, for when he makes 
a complete turn of the spiral he must, in the course 
of it, have the wind blowing first on one side of him 
and afterwards on the other. But the Gull too does 
something that approaches to this, for when he 
glides sideways he is not absolutely full face to the 
wind, but makes a slight turn so that it strikes him 
a little on one side. The soaring Eagle faces each 
point of the compass in turn, for he has to circle 
round in order not to pass beyond the limits of the 
upward stream of air that supports him. Let us 
picture him as he turns the spiral. He does not, 
like the earth, keep his axis pointed the same way 
from the beginning to the end of his orbit. As he 

* Loc. cit., p. 88. 


150 


THE FLIGHT OF BIRDS 


revolves, he faces north, west, south, east in turn, 
or vice versa ; and, as he turns, the inner wing will 
always be rather lower than the outer one—this is 
with all birds the commonest way of steering. 
How he changes his balance is not quite clear. He 
may, holding his wings rigidly in a straight line, 
pull his body towards one wing or the other, and so 
weight one side more heavily than the other. More 
probably he bends sideways at the waist, as birds 
certainly do for balancing and steering purposes 
(see p. 60). Whatever the method, he has to execute 
slow, swaying movements with the utmost skill. 
Only when he travels straight for a bit will the two 
wings be on the same level. There is no reason why 
he should be in difficulties at all when the wind 
strikes him on the side. It will support him which¬ 
ever way his head points. A glance at the frontis¬ 
piece will help to make matters clear. Though the 
wings are held rigid, they are seldom horizontal for 
long together. The spiral movement demands a 
continual swaying of the body ; for the correct 
incline, not only fore and aft, but also from left to 
right, has to be maintained. The tail is frequently 
at work, and this is probably accompanied by the 
other movements I have just described. Otherwise 
the body would not swing round promptly ; the bird 
would not be able to describe his airy spirals. Fre¬ 
quently the wings are held slanting throughout the 
“ circle,” the inner wing pointing downwards to the 
centre and seeming to act as a pivot on which the 
bird revolves. But this is not always the case. 
Sometimes a bird will put in a considerable bit of 
straight-ahead travelling, gaining, maintaining or 


WIND AND FLIGHT 151 

losing altitude as he goes. Not unfrequently birds 
change from a left-handed to a right-handed spiral; 
Gulls are very fond of this ; they often describe 
very small “ circles,” revolving round the down¬ 
ward-pointing inner-wing, and it may be that the 
change to the right-handed spiral saves them from 
giddiness. 

Mr. Peal was of opinion that the Adjutants, whose 
soaring he studied so zealously, always made leeway 
as they rose. But there seems to be no reason why 
this should be so. It may have been that what 
looked like a movement to leeward had for its object 
the keeping within an upward slanting stream of air 
on which they depended. There seems no reason 
for an involuntary loss of leeway. Let us consider 
a particular turn of the spiral from start to finish. 
When the bird is facing the wind, he has only to 
incline his body correctly and he will make headway. 
When the wind strikes him from behind, he must 
slope his body downward, otherwise there will be a 
disarrangement of plumage, and there will result a 
slight loss by leeway, though no loss of elevation 
unless the bird so chooses. When he turns his side 
to the wind he is free to set himself at any incline he 
likes, and, according as he chooses, he will advance 
or retreat. During far the greater part of the circle, 
therefore, he is far from being the plaything of the 
wind. As a matter of fact birds may be seen circling 
over hill-tops without any loss by leeway, and I 
believe I have seen them equally successful over 
plains. There is this, too, to be remembered. When 
a bird has gained all the altitude he wants, by gliding 
slightly downward he can make headway in whatever 


152 


THE FLIGHT OF BIRDS 

direction he wants, and, if there has been any loss by 
leeway, make it good. 

Soaring in a Horizontal Wind impossible. 

Given two things, a strong upward stream of air 
and a big bird possessed of great skill, soaring 
becomes quite explicable. But there are still some 
people, I believe, who hold that an up-current 
is not an indispensable condition, but that a hori¬ 
zontal wind is all that is needed if only the wind is 
not uniform, so that somehow the bird may perpetu¬ 
ally manage to be passing from a comparatively 
slow current into a faster one. How such a process 
can be continued for an indefinite time is more than 
I can understand. Nevertheless some great mathe¬ 
maticians, who do not, however, profess to have 
actually watched birds soaring, maintain that it 
is theoretically possible. Setting aside theoretic 
possibilities for the moment, let us see what we can 
learn by observation. Wind is least uniform close 
to the earth, and there we find birds turning this 
want of uniformity to account. They face the wind 
as they rise and get help from it, owing to the fact 
that they are perpetually passing from comparatively 
slowly moving air into a more rapid current. But 
they cannot get the wind to do the whole work of 
lifting, whatever onward momentum they may have. 
They ply their wings vigorously all the time. 

Evidently, then, a wind with varying velocities is 
not enough to account for soaring. Imagine, too, 
what would happen as the bird circled round. In 
each complete turn of the helix he must, for part of 
the time, have his back turned to the wind, and the 


WIND AND FLIGHT 


153 


wind impinging on it—I assume the bird’s body 
would be inclined slightly upward—would drive the 
bird downward, reinforcing gravitation with fatal 
effect. Again, it has been imagined that he is 
somehow perpetually passing from a slowly moving 
to a rapidly moving part of an eddy, a very precarious 
business. Even if we grant that such a method of 
soaring is theoretically possible, yet the stately, 
sedate wheeling of an Eagle shows that he is depen¬ 
dent on quite different conditions. He finds a strong 
up-current at his service, and he is securely riding 
upon it. Practical aviators, if they have not already 
done so, will be interested to read Mr. Wilbur 
Wright’s observations on soaring birds. He has no 
doubt as to the necessity of an up-current. He has 
often watched Buzzards soaring ; he calculated the 
upward incline of the wind where it was deflected 
by a hill, the hill where he and his brother were 
practising with their glider, and he considered the 
question whether he might not himself with practice 
learn to soar.* 

And now wonderful news has come from America. 
Mr. Orville Wright has ascended on his glider, lifted 
by an up-current, and for sixty seconds has hung, 
almost without a quiver, in the air at a height of 
seventy feet over a hill-top, a truly marvellous 
achievement, worthy of a Kestrel. 


* See u Gliding Experiments of the Wright Brothers,” in the 
Aero Manual, 1910. 


CHAPTER XI. 


SOME ACCESSORIES. 

DIGESTION—CIRCULATION, BREATHING, TEMPERATURE—REPAIR 
OF THE MACHINE—CALL-NOTES AND SONG. 

No one who undertook to describe a steam-engine 
would ever dream of omitting all mention of the 
furnace. To do so would be to leave out, if not the 
part of Hamlet, yet a most important part. I have, 
therefore, determined to write a short chapter on 
certain “ accessories,” which would, perhaps, be 
more correctly called preliminaries. These pre¬ 
liminaries are matters of physiology, such as feeding, 
digestion, breathing, regulation of temperature. 

Digestion. 

A bird is a glorified reptile, and one great difference 
between him and his cold-blooded ancestors and his 
cold-blooded surviving relatives also is that he needs 
far more fuel to keep the flame of his life burning 
with its normal brightness than they require to 
maintain their slowly smouldering fires. The bird 
has a huge appetite, and, except when a demand 
for a prolonged, uninterrupted effort is made, he 
craves for ample meals at no long intervals. The 
boa-constrictor will go a week, or, in captivity, three 
weeks or more without eating, even during hot 
weather. But such abstinence does not suit a bird. 


SOME ACCESSORIES 


155 


He has a temperature considerably higher than that 
of a human being ; in some species it ranges up to 
111° F., or even just over that. In fact his vitality 
is very great, and all his physiological processes are 
brisk and vigorous. It would not do, therefore, to 
economise in fuel. Though his fore limbs have been 
metamorphosed into wings and are incapable of 
doing the work of hands, his beak at the end of his 
long, supple neck (for it is of some length even in a 
comparatively short-necked bird) is quite equal to 
the task, and picks up big and small things easily 
and with skill. Moreover, birds of prey use their 
feet as hands, and most effectively too. As soon 
as he has seized his food, the bird sends it post-haste 
to his crop, or, if he is cropless, to his stomach, his 
proventriculus, to be digested. He has nothing cor¬ 
responding to the apparatus that makes breathing 
easy for us while we chew our food. Nor has he 
any teeth ; powerful teeth could only exist if his 
jaws were strong and heavy, and it is important 
that his head should be light, for a heavy head would 
make fore-and aft balance difficult during flight. 
Digestion is rapid. In the case of a seed-eater, after 
the digestive juices have operated on the food in the 
proventriculus, it passes to the gizzard, to be ground 
up in that powerful mill. There seems to be no 
period of torpor after a meal as there is with a reptile, 
though the actual weight of what is swallowed may 
make flight difficult. 

Circulation, Breathing, Temperature. 

The heart is very efficient. Like the mammalian 
heart, it has four chambers, with an impassable wall 


156 


THE FLIGHT OF BIRDS 


separating the two on the right from the two on the 
left, and the arterial blood is kept quite apart from 
the venous. The valve between the upper and lower 
chambers on the right is a single flap of muscle, very 
unlike the three flaps of membrane found in the 
mammal’s heart. But it works in the same way, and 
is no less efficient. The blood swarms with red 
corpuscles, not round and without nucleus like ours, 
but oval and nucleated. Where they come from 
is a question. The marrow in human bones is 
believed to be a factory of red corpuscles, and so we 
cannot help wondering what substitute for this those 
birds may have whose chief bones are hollow, with 
only the very thinnest lining of marrow We know 
that they have somewhere as good a factory as any 
mammal has ; the richness in corpuscles banishes 
all doubt on this point. It may be that the spleen 
is very active. It is known that in mammals during 
the embryo stages and in after-life, in emergencies, 
the spleen gives birth to many red corpuscles. 
Whatever their origin may be, there they are in their 
thousands, putting vitality and energy into the bird. 

The breathing apparatus is as wonderful as any 
part of this living flying machine. The lungs are 
very small and may be seen, brilliantly scarlet, 
neatly packed on either side of the backbone. Their 
efficiency is not to be measured by their very diminu¬ 
tive size. They have large extensions called air-sacs, 
into which the air rushes, passing through the lungs, 
when inspiration takes place. Some of the air 
breathed in finds its way at once into the minute 
air-passages that ramify about the lungs and does the 
work of oxidising the blood. The rest rushes straight 



SOME ACCESSORIES 


157 

through large channels to the air-sacs, and, when 
expiration takes place, passes again, still almost 
fresh, to the lungs, and some of it, getting from the 
main passage to the little ramifications, continues the 
process of oxidation. Thus, if the bird takes twenty 
breaths in a minute, the lungs are supplied forty times 
with fresh air. If he has hollow bones—and in some 
birds almost all are hollow—pouches of pulmonary 
membrane extend into the cavities, and thus the 
bones are filled with air from the lungs (see p. 77). 

When a bird is standing his breastbone moves with 
each breath ; in a captive bird this is easy to see. 
In the case of Pelicans I have found the rate per 
minute to vary from 5-11, from very slow to rather 
slow, while in the most rapid breather, a Canary, 
it was not far short of 100. Between these 
extremes came a Blackbird with 39, a Bulbul with 
48, an Ouzel with, at one time, 34, at another 50. 
Evidently the big bird when at rest is a slow breather; 
a Griffon Vulture took only nine respirations. The 
small birds are the more rapid, and as a rule they have 
a higher temperature. The Great Tit and the Swift 
are at one extreme with 111*2° F., and the Ostrich 
at the other with 99*2°. In between come the Duck 
with 109*1° and the Heron with 105*8°. The slow 
breathing of the big birds during rest is remarkable, 
but we cannot doubt that it becomes rapid during 
flight, whether the bird be big or small. Unfortu¬ 
nately it is very difficult to make observations, but 
of the method of breathing we may get some idea 
by watching a bird lying on his breast When he 
adopts this attitude it is easy to see that his back 
rises with each respiration, no movement being 


158 


THE FLIGHT OF BIRDS 


possible for his breast since he is resting his weight 
upon it. There is every reason to believe that he 
breathes in the same way during flight, his backbone 
rising and falling while his breastbone remains 
steady. It is difficult for the breastbone to move 
freely, since the pressure inwards of the wings tends 
to hold it fixed. Were it to move easily it would 
form a very unsteady pivot for the wings. The 
lowering of the wing helps to lift the back, for as it 
descends it hauls upon a muscle which passes from 
the upper armbone to the backbone, and sometimes 
even to the pelvis. Attempting to get direct evi¬ 
dence of this method of breathing, I suspended a 
freshly-killed pigeon by its wings and inflated its 
air-sacs by means of a blowing-tube inserted in the 
windpipe. The backbone, a little anterior to the 
thigh-joint, moved rather more than half an inch, the 
movement of the breastbone being almost too slight 
to measure. Of course, the conditions that obtain 
during flight were not reproduced ; there was no 
pressure inwards. But the only result of such 
pressure would be to render the breastbone and the 
bones united with it still less ready to move. 

The spacious air-sacs are useful not only for 
breathing. The bird regulates his temperature, but 
not by the machinery that is most effective in human 
beings and most mammals, for, like his reptilian 
ancestors, he does not perspire at any part of his 
surface. During hard exercise he prevents a rise to 
fever heat by giving off aqueous vapour from his 
lungs, and besides this the great amount of air that 
he breathes out when respiration is rapid has pre¬ 
sumably a temperature not much below that of the 


159 


SOME ACCESSORIES 

body. In other words, he is parting rapidly with 
caloric, and the result is that his temperature is 
regulated as effectively as any mammal’s, though 
the system is in the main unlike that which operates 
in ourselves. All mammals, of course, throw off 
aqueous vapour from the lungs, but in most of them 
perspiration plays a very large part. Before passing 
on to other matters we must note that the air in 
pneumatic bones can be of little or no service in 
breathing, since it cannot be expelled at will. 

Repair of the Machine. 

Very often unfair comparisons are drawn between 
nature’s machines and those which men manufacture. 
It is forgotten that the former have not only to do 
their special work, but also to keep themselves in 
repair ; besides which they must reproduce them¬ 
selves, i.e. they must be practically immortal. What 
a contrast to this is presented by an aeroplane which, 
without the constant attendance of skilled artificers, 
is speedily reduced to helplessness! Of the bird’s 
stoking I have already spoken. The repairing is 
very wonderfully effected (see pp. 88, 89) without 
the machine having to go into hospital. The great 
wing-feathers on which flight depends and those in 
the tail also are moulted in pairs, so that, though 
not at his best while the process is going on, the bird 
is at no time incapable of flight. A few birds, as I 
have pointed out above, are exceptions, but under 
their special circumstances flightlessness, though, 
no doubt, an inconvenience, does not as a rule bring 
disaster. But for all warm-blooded beasts there is, 
of course, a time of helplessness when they sleep and 


160 


THE FLIGHT OF BIRDS 


recuperate after the labours of the day. For most 
birds, however, the danger is reduced to a minimum. 
While they sleep perched on a branch the weight of 
their bodies keeps their legs bent at the ankle, and 
when the ankle is bent the toes grip automatically. 
Were it not for the automatic grip of the toes, the 
bird would fall and be at the mercy of any prowling 
enemy. 

Call-notes and Song. 

When the bird became able to move with speed and 
make long journeys, some new power was required 
to bring the sexes together and to prevent a pair 
that had mated from becoming separated. Hence 
it is that birds are loud-voiced, though reptiles are 
nearly all of them dumb. To a mere call-note, a 
very humble origin, it is probable that we can trace 
the beautiful songs of the Nightingale and the 
Blackcap. For many of the migrant birds in par¬ 
ticular a loud, easily recognisable call-note was an 
urgent need. After a flight of some thousands of 
miles cock and hen must somehow find each other. 
The Nightingale, when he has passed the Mediter¬ 
ranean and reached his northern home, trumpets 
forth to all the world the fact that he has arrived. 
His future mate hears the call, and together they set 
about the all-important business of nesting. And 
thus we see that all a bird's activities must be viewed 
in connection with the fact that he has wings and 
is capable of long flights. 


The End. 


INDEX. 


Adjutant Bird, 68,142 
Aero Manual, 18 
Aeroplanes, 10, 12, 16, 18, 27, 
36, 54, 61 
Air, Rarefied, 112 

-Resistance of, 2-5, 6-22, 41 

Albatross, 137-139 
Alighting, 64-66 (PI. x), 124 
Alix, M., 82 

Angle of Incline, see Friction, 
Gliding, Stability, Tacking 
Archaeopteryx, 95 
Area and Cubic Content, 19, 76 
Aston, Mr. W. G., 18 
Avanzini, Law of, 26 
Aviators, see Aeroplanes 

Backbone, 60, 62 

Beetham, Mr. Bentley, 61, 65 

Big Birds, 45, 55-58, 97, 98, 143, 

144 

Blood Corpuscles, 156 
Bones, see Clavicle, etc. 

-Pneumatic, 76-78 

Bonhote, Mr. J. L., 89, 118 
Breastbone, 67-69 
Breathing, 157 

Brooke-Popham, Captain, 54, 
146, 149 

Buller, Mr. W. L., 110 
Butterfly, 19 
Buzzard, 153 

Camber, see Curve 
Carpals, 80 

Carpenter, Mr. F. W., 114 
Catapult (used to illustrate Glid¬ 
ing), 26 

Cave, Mr. C. J., 146 
Chapman, Mr. Abel, 101, 119 

-Mr. F. M., 114 

Circulation of Blood, 155 


Clavicle, 69, 70 

Clayton, Mr. H., 105 

Cody, Mr., 61 

Condor, 52 

Coracoid, 69 

Cormorant, 66, 121 

Cubic Content and Area, 19, 76 

Cuckoos in New Zealand, 110 

Curlew (see PI. xiii), 32, 93 

Curve of Wings, 15-18 

-Excessive, 25, 26 

Darwin, 52 
de Lucy, M., 19 
Digestion, 154 

Duck, 31, 32, 53, 62, 68, 98, 119, 
143 

Eagle, 57, 77, 136 
Endurance, 117-119 
Equilibrium (see Stability), 23-32 

Falcon, 68 

Feathers (see Pis. xi and xii), 
46, 84-86 

-Moulting of, 88, 89 

-Rotation of, 82 

-Structure of, 86, 88 

Feet, Use of, in Flight, 31, 32 

-Structure of, 90 

-Webbed, 60 

Flamingo, 20, 31 
Flight, Direct Advance without 
Wing-strokes, 135 (PI. xvi), 

-in Flocks, 101 

-Sideways, without Wing- 

strokes, 139 

-Styles of, 31, 96-102 

-Undulating without Wing- 

strokes, 130 

-Velocity of, 103-117 

-with the Wind, 124-130 












162 


INDEX— continued. 


Flight (Aero Weekly), 14, 16, 25 
Fowl, Jungle, 73 
Friction, 14 

Gannet (see PI. xiv), 65, 76, 85, 
94, 121, 133 
Gilson, Mr. R. C., 147 
Glaisher, Mr., 112 
Gliders (see Aeroplanes), 25, 26, 
29 153 

Gliding, Birds, 1-22, 96, 97 
Gnat, 19 

Godwit, Eastern, 111 
Goose, 88 

Gravity, Centre of, 23, 24 
Grouse, 73, 98, 100 
Guillemot, 68 
Gulls, 47, 60 (PI. ix.), 77 

-Play of, 128 

-Soaring of, 145 

-Progression of, without 

Wing-strokes, 130-132, 139- 
141 (PL xvi) 

Hawks, 33, 74, 98 
Heart, 156 
Helmholtz, 56 
Heron, 31, 32, 50, 117 
Hoatzin, 68 
Hoopoe, 92 
Hornhill, 78 
Humerus, 71 
Hutton, Captain, 110 

Jay, 91, 93 (PI. xv) 

Kestrel, 4, 41, 146 
Kite-flying, 9 
Kites Soaring, 146 

Lancaster, C., 104 
Langley, Prof., 12, 13, 125 
Lapwing (PI. xiv), 32, 64, 66, 98 
Legal and Reichel, 24, 32 
Legs, Use of, in Balancing, 31, 
32, 60 

-Structure of, 90 

Lift and Drift, 1-15 
Ligament, Elastic, 82 (PI. xi) 
Lilienthal, 16 


Lizard, 81 

Lynes, Commander, 104, 126 

Marey, Prof., 4, 5, 19, 24, 40, 43, 
50, 144 

Marriott, Mr. W., 113 
Martin, House-, 35 
Maxim, Sir H., 12, 145 
Membranes, Wing, 85 
Metacarpals, 80 
Migration, 109-117, 160 
Moor-hen, 75 
Moulting, 88, 89, 159 
Muscle, Quality of, 46, 55, 72-75, 
81 

Muscles 45, 55, 70-72 
-Weight of, 23 

Newton, Sir I., 3, 12 
Nuthatch, 65 

Ouzel, Water-, 100 
Owl, 102 

Pace, 103-117 
Parallelogram of Forces, 7 
i Payne-Gallwey, Sir R., 104 
I Peal, Mr. S. E., 142, 151 
] Pettigrew, Prof., 119 
Pheasant, 53, 92 (PI. xiii.) 

Pigeon (see Pis. ii, iii, iv, v, vi, 
viii, ix, x), 19, 32, 46, 64, 
97, 103 

-Homing, 105, 106, 117 

Pilcher, Mr., 29 

Plover, American Golden, 109 

Pressure, Centre of, 34 

Pterodactyle, 49 

Puffin, 48 

Pygostyle, 33 

Racehorse, The, 75 
Radius, 80, 82 
Rails, 89 
Raven, 146 

Reed-Warbler, Clamorous, 92 
(PI. xv) 

Robertson, Mr. J. B., 75 
! Rook, 129 








INDEX— continued. 


163 


Scapula, 69 

Shearwater, 133, 134, 139 
Snipe, 130 
Soaring, 141-153 

-among Mountains, 144 

-always over Land, 145 

-over Plains, 146 

-impossible for small Birds, 

143 

-impossible in horizontal 

wind, 152 

-spiral Movement in, 148, 

151 

Sparrow-Hawk (see PI. xiii), 
32 

Starling, Prof., 73 

-93 (PI. xv) 

Stability, 23-37 

-Pendulum, 25 

Starting, 48-58 (PI. viii), 121- 
124 

Steering, 59-63 (PI. ix) 

Sternum, 67-69 
Stork, 19, 20, 101 
Surface, Area of supporting, 18- 
22 

Swallow, 19, 20, 98, 106, 128 

-Australian, 111 

Swift, 35, 48 


Tacking, Boat, 8, 26j 

Tail, use of, in Balancing, 31-34 

-Soaring, 150 

-Steering, 60 

Temperature, Regulation of, 
159 

Tern, 64, 94 
Thienemann, Dr., 107 


Ulna, 80 

Vane, Experiment with, 134 
Velocity, see Flight. 

Vertebrae, 76 

Voluntary Adjustments, 29-37 
Vulture, 144 

Weight and Supporting Surface, 
19 

Wind and Flight, 107-109, 120- 
153 

Wind, at High Altitudes, 112- 
117 

-Horizontal, 130, 152 

-not Uniform in Velocity, 

122-124, 125 

-having an upw T ard Trend, 

130-153 

Wing (see Area, Surface, Sta¬ 
bility), Curve of, 15-18, 25, 
26 

-Elasticity of, 27 

-as Lever, 38-40 

-Rotation of, 51-54 

-Shape of (see Pis. xiii- 

xv), 91-96 

-Spreading of, 81-86 

-Weight of, 75 

Wings, Whir of, 101 
Wing-strokes, Phases of (see Pis. 
iv.-vii.), 40-47, 84, 85 

-Rate of, 50, 99, 144 

-Short, 46 

-Unequal (Pi. ii), 30 

Witherby, Mr. H. F., 66 
Woodpecker, 55, 65 
Wright, Mr. Wilbur and Mr. 
Orville, 16, 153. 




























































































































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